Massive JavaScript cleanup

- Use JSX for templates
- Docblock/comment everything
- Mostly passes ESLint (still some work to do)
- Lots of renaming, refactoring, etc.

CSS hasn't been updated yet.

js/forum/src/components/DiscussionsSearchSource.js

@@ -0,0 +1,55 @@
+import highlight from 'flarum/helpers/highlight';
+import Button from 'flarum/components/Button';
+
+/**
+ * The `DiscussionsSearchSource` finds and displays discussion search results in
+ * the search dropdown.
+ *
+ * @implements SearchSource
+ */
+export default class DiscussionsSearchSource {
+  constructor() {
+    this.results = {};
+  }
+
+  search(query) {
+    this.results[query] = [];
+
+    const params = {
+      filter: {q: query},
+      page: {limit: 3},
+      include: 'relevantPosts,relevantPosts.discussion,relevantPosts.user'
+    };
+
+    return app.store.find('discussions', params).then(results => this.results[query] = results);
+  }
+
+  view(query) {
+    const results = this.results[query] || [];
+
+    return [
+      <li className="dropdown-header">Discussions</li>,
+      <li>
+        {Button.component({
+          icon: 'search',
+          children: 'Search all discussions for "' + query + '"',
+          href: app.route('index', {q: query}),
+          config: m.route
+        })}
+      </li>,
+      results.map(discussion => {
+        const relevantPosts = discussion.relevantPosts();
+        const post = relevantPosts && relevantPosts[0];
+
+        return (
+          <li className="discussion-search-result" data-index={'discussions' + discussion.id()}>
+            <a href={app.route.discussion(discussion, post && post.number())} config={m.route}>
+              <div className="title">{highlight(discussion.title(), query)}</div>
+              {post ? <div className="excerpt">{highlight(post.contentPlain(), query, 100)}</div> : ''}
+            </a>
+          </li>
+        );
+      })
+    ];
+  }
+}