Repurpose React app as artifact editor (#89)

Summary:
We’ll now start creating the artifact plugin. A large part of this will
be the user interface, including a GUI. For now, our build system just
builds a single React app, so we’re cannibalizing the main explorer to
serve this purpose.

Paired with @dandelionmane.

Test Plan:
The following still work:
  - `yarn test`
  - `yarn start`
  - `yarn build; (cd build; python -m SimpleHTTPServer)`

wchargin-branch: repurpose-react-app-as-artifact-editor

config/paths.js

@@ -42,9 +42,9 @@ function getServedPath(appPackageJson) {
 module.exports = {
   dotenv: resolveApp(".env"),
   appBuild: resolveApp("build"),
-  appPublic: resolveApp("public"),
+  appPublic: resolveApp("src/plugins/artifact/editor/public"),
-  appHtml: resolveApp("public/index.html"),
+  appHtml: resolveApp("src/plugins/artifact/editor/public/index.html"),
-  appIndexJs: resolveApp("src/explorer/index.js"),
+  appIndexJs: resolveApp("src/plugins/artifact/editor/index.js"),
   appPackageJson: resolveApp("package.json"),
   appSrc: resolveApp("src"),
   yarnLockFile: resolveApp("yarn.lock"),