Warn if base !== master, tag CODEOWNERS

Summary:
The following PR modifies the Danger rules in the following way:

1. Verifies if a PR is opened against master. If not, it will warn (if opened against stable) or fail (anything else).
2. No longer adds a markdown message tagging the facebook/react-native team, as the bot does not have the necessary scope to mention the team.
3. Mentions people that have marked themselves as interested in a file, when that file is modified. This is based off CODEOWNERS. The bot should be able to use mentions here as it will act as any other regular user.

Verify it tags the right people in https://github.com/facebook/react-native/pull/15139

```
$ npm run danger pr https://github.com/facebook/react-native/pull/15139

> @ danger /Users/hramos/git/react-native/danger
> node ./node_modules/.bin/danger "pr" "https://github.com/facebook/react-native/pull/15139"

{
  fails: [],
  warnings: [],
  messages: [],
  markdowns: ["Attention: grabbou, kureev"]
}
```

It should not tag anyone for https://github.com/facebook/react-native/pull/15175:

```
$ npm run danger pr https://github.com/facebook/react-native/pull/15175

> @ danger /Users/hramos/git/react-native/danger
> node ./node_modules/.bin/danger "pr" "https://github.com/facebook/react-native/pull/15175"

{
  fails: [],
  warnings: [],
  messages: [],
  markdowns: []
}
```

It should warn on https://github.com/facebook/react-native/pull/14640 as it targets 0.45-stable:

```
$ npm run danger pr https://github.com/facebook/react-native/pull/14640

> @ danger /Users/hramos/git/react-native/danger
> node ./node_modules/.bin/danger "pr" "https://github.com/facebook/react-native/pull/14640"

{
  fails: [],
  warnings: [
    {
      message: "❔ Base Branch - <i>The base branch for this PR is something other than `master`. Are you sure you want to merge these changes into a stable release? If you are interested in backporting updates to an older release, the suggested approach is to land those changes on `master` first and then cherry-pick the commits into the branch for that release. The [Releases Guide](https://github.com/facebook/react-native/blob/master/Releases.md) has more information.</i>"
    }
  ],
  messages: [],
  markdowns: ["📄 Thanks for your contribution to the docs!"]
}
```

It should not warn on https://github.com/facebook/react-native/pull/15175 because it targets master.
```
$ npm run danger pr https://github.com/facebook/react-native/pull/15175

> @ danger /Users/hramos/git/react-native/danger
> node ./node_modules/.bin/danger "pr" "https://github.com/facebook/react-native/pull/15175"

{
  fails: [],
  warnings: [],
  messages: [],
  markdowns: []
}
```
Closes https://github.com/facebook/react-native/pull/15179

Differential Revision: D5490047

Pulled By: hramos

fbshipit-source-id: a46a23b7d0a59d12b8039746d6e9c4399ef32d5f

danger/dangerfile.js

@@ -11,6 +11,7 @@
 
 const fs = require('fs');
 const includes = require('lodash.includes');
+const minimatch = require('minimatch');
 
 import { danger, fail, markdown, warn } from 'danger';
 
@@ -33,7 +34,6 @@ if (addsBlogPost) {
   const idea = 'This PR appears to add a new blog post, ' +
     'and may require further review from the React Native team.';
   warn(`${message} - <i>${idea}</i>`);
-  markdown(':memo: This PR requires attention from the @facebook/react-native team.');
 }
 
 // Flags edits to blog posts
@@ -43,7 +43,6 @@ if (editsBlogPost) {
   const idea = 'This PR appears to edit an existing blog post, ' +
     'and may require further review from the React Native team.';
   warn(`${message} - <i>${idea}</i>`);
-  markdown('This PR requires attention from the @facebook/react-native team.');
 }
 
 // Fails if the description is too short.
@@ -66,7 +65,6 @@ if (packageChanged) {
   const idea = 'Changes were made to package.json. ' +
     'This will require a manual import by a Facebook employee.';
   warn(`${message} - <i>${idea}</i>`);
-  markdown('This PR requires attention from the @facebook/react-native team.');
 }
 
 // Warns if a test plan is missing.
@@ -102,3 +100,37 @@ if (issueCommandsFileModified) {
     'GitHub bot.';
   warn(`${message} - <i>${idea}</i>`);
 }
+
+// Warns if the PR is opened against stable, as commits need to be cherry picked and tagged by a release maintainer.
+// Fails if the PR is opened against anything other than `master` or `-stable`.
+const isMergeRefMaster = danger.github.pr.base.ref === 'master';
+const isMergeRefStable = danger.github.pr.base.ref.indexOf(`-stable`) !== -1;
+if (!isMergeRefMaster && isMergeRefStable) {
+  const message = ':grey_question: Base Branch';
+  const idea = 'The base branch for this PR is something other than `master`. Are you sure you want to merge these changes into a stable release? If you are interested in backporting updates to an older release, the suggested approach is to land those changes on `master` first and then cherry-pick the commits into the branch for that release. The [Releases Guide](https://github.com/facebook/react-native/blob/master/Releases.md) has more information.';
+  warn(`${message} - <i>${idea}</i>`);
+} else if (!isMergeRefMaster && !isMergeRefStable) {
+  const message = ':exclamation: Base Branch';
+  const idea = 'The base branch for this PR is something other than `master`. [Are you sure you want to target something other than the `master` branch?](http://facebook.github.io/react-native/docs/contributing.html#pull-requests)';
+  fail(`${message} - <i>${idea}</i>`);
+}
+
+// People can add themselves to CODEOWNERS in order to be automatically added as reviewers when a file matching a glob pattern is modified. The following will have the bot add a mention in that case.
+const codeowners = fs.readFileSync('../.github/CODEOWNERS', 'utf8').split('\n');
+let mentions = [];
+codeowners.forEach((codeowner) => {
+  const pattern = codeowner.split(' ')[0];
+  const owners = codeowner.substring(pattern.length).trim().split(' ');
+
+  const modifiedFileHasOwner = path => minimatch(path, pattern);
+  const modifiesOwnedCode = danger.git.modified_files.filter(modifiedFileHasOwner).length > 0;
+
+  if (modifiesOwnedCode) {
+    mentions = mentions.concat(owners);
+  }
+});
+const isOwnedCodeModified = mentions.length > 0;
+if (isOwnedCodeModified) {
+  const uniqueMentions = new Set(mentions);
+  markdown('Attention: ' + [...uniqueMentions].join(', '));
+}

danger/package.json

@@ -5,6 +5,7 @@
   },
   "devDependencies": {
     "danger": "^0.21.2",
-    "lodash.includes": "^4.3.0"
+    "lodash.includes": "^4.3.0",
+    "minimatch": "^3.0.4"
   }
 }