Fix missing prefixes in Discourse plugin (#1309)

Summary:
A `PluginDeclaration` must have a `nodePrefix` and an `edgePrefix`, but
the Discourse plugin declaration was missing these. This was not caught
by Flow because `deep-freeze` was introduced in #1249 without type
definitions; see #1308.

Test Plan:
Apply the following patch:

```diff
diff --git a/src/plugins/discourse/declaration.js b/src/plugins/discourse/declaration.js
index 246a0a28..36ae5f13 100644
--- a/src/plugins/discourse/declaration.js
+++ b/src/plugins/discourse/declaration.js
@@ -1,6 +1,6 @@
 // @flow

-import deepFreeze from "deep-freeze";
+declare function deepFreeze<T>(x: T): T;
 import type {PluginDeclaration} from "../../analysis/pluginDeclaration";
 import type {NodeType, EdgeType} from "../../analysis/types";
 import {NodeAddress, EdgeAddress} from "../../core/graph";
```

Note that, with this patch, `yarn flow` fails before this change but
passes after it. Running `yarn unit` still passes.

wchargin-branch: discourse-plugin-prefixes

src/plugins/discourse/declaration.js

@@ -5,10 +5,13 @@ import type {PluginDeclaration} from "../../analysis/pluginDeclaration";
 import type {NodeType, EdgeType} from "../../analysis/types";
 import {NodeAddress, EdgeAddress} from "../../core/graph";
 
+export const nodePrefix = NodeAddress.fromParts(["sourcecred", "discourse"]);
+export const edgePrefix = EdgeAddress.fromParts(["sourcecred", "discourse"]);
+
 export const topicNodeType: NodeType = deepFreeze({
   name: "Topic",
   pluralName: "Topics",
-  prefix: NodeAddress.fromParts(["sourcecred", "discourse", "topic"]),
+  prefix: NodeAddress.append(nodePrefix, "topic"),
   defaultWeight: 2,
   description:
     "A topic (or post-container) in a Discourse instance. Every topic has at least one post.",
@@ -17,7 +20,7 @@ export const topicNodeType: NodeType = deepFreeze({
 export const postNodeType: NodeType = deepFreeze({
   name: "Post",
   pluralName: "Posts",
-  prefix: NodeAddress.fromParts(["sourcecred", "discourse", "post"]),
+  prefix: NodeAddress.append(nodePrefix, "post"),
   defaultWeight: 1,
   description: "A post in some topic in a Discourse instance.",
 });
@@ -25,7 +28,7 @@ export const postNodeType: NodeType = deepFreeze({
 export const userNodeType: NodeType = deepFreeze({
   name: "User",
   pluralName: "Users",
-  prefix: NodeAddress.fromParts(["sourcecred", "discourse", "user"]),
+  prefix: NodeAddress.append(nodePrefix, "user"),
   defaultWeight: 1,
   description: "A user account on a particular Discourse instance.",
 });
@@ -33,11 +36,7 @@ export const userNodeType: NodeType = deepFreeze({
 export const topicContainsPostEdgeType: EdgeType = deepFreeze({
   forwardName: "contains post",
   backwardName: "is contained by topic",
-  prefix: EdgeAddress.fromParts([
-    "sourcecred",
-    "discourse",
-    "topicContainsPost",
-  ]),
+  prefix: EdgeAddress.append(edgePrefix, "topicContainsPost"),
   defaultWeight: {forwards: 1 / 16, backwards: 1 / 4},
   description: "Connects a topic to the posts that it contains.",
 });
@@ -45,7 +44,7 @@ export const topicContainsPostEdgeType: EdgeType = deepFreeze({
 export const postRepliesEdgeType: EdgeType = deepFreeze({
   forwardName: "post is reply to",
   backwardName: "post replied to by",
-  prefix: EdgeAddress.fromParts(["sourcecred", "discourse", "replyTo"]),
+  prefix: EdgeAddress.append(edgePrefix, "replyTo"),
   defaultWeight: {forwards: 1, backwards: 1 / 16},
   description: "Connects a post to the post that it is a reply to.",
 });
@@ -53,12 +52,7 @@ export const postRepliesEdgeType: EdgeType = deepFreeze({
 export const authorsTopicEdgeType: EdgeType = deepFreeze({
   forwardName: "authors topic",
   backwardName: "topic is authored by",
-  prefix: EdgeAddress.fromParts([
-    "sourcecred",
-    "discourse",
-    "authors",
-    "topic",
-  ]),
+  prefix: EdgeAddress.append(edgePrefix, "authors", "topic"),
   defaultWeight: {forwards: 1 / 4, backwards: 1},
   description: "Connects an author to a topic they created.",
 });
@@ -66,7 +60,7 @@ export const authorsTopicEdgeType: EdgeType = deepFreeze({
 export const authorsPostEdgeType: EdgeType = deepFreeze({
   forwardName: "authors post",
   backwardName: "post is authored by",
-  prefix: EdgeAddress.fromParts(["sourcecred", "discourse", "authors", "post"]),
+  prefix: EdgeAddress.append(edgePrefix, "authors", "post"),
   defaultWeight: {forwards: 1 / 4, backwards: 1},
   description: "Connects an author to a post they've created.",
 });
@@ -74,13 +68,15 @@ export const authorsPostEdgeType: EdgeType = deepFreeze({
 export const likesEdgeType: EdgeType = deepFreeze({
   forwardName: "likes",
   backwardName: "is liked by",
-  prefix: EdgeAddress.fromParts(["sourcecred", "discourse", "likes"]),
+  prefix: EdgeAddress.append(edgePrefix, "likes"),
   defaultWeight: {forwards: 1, backwards: 1 / 16},
   description: "Connects a Discourse user to a post they liked.",
 });
 
 export const declaration: PluginDeclaration = deepFreeze({
   name: "discourse",
+  nodePrefix,
+  edgePrefix,
   nodeTypes: [userNodeType, topicNodeType, postNodeType],
   edgeTypes: [
     postRepliesEdgeType,