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/PostPreview.js

@@ -0,0 +1,32 @@
+import Component from 'flarum/Component';
+import avatar from 'flarum/helpers/avatar';
+import username from 'flarum/helpers/username';
+import humanTime from 'flarum/helpers/humanTime';
+import highlight from 'flarum/helpers/highlight';
+
+/**
+ * The `PostPreview` component shows a link to a post containing the avatar and
+ * username of the author, and a short excerpt of the post's content.
+ *
+ * ### Props
+ *
+ * - `post`
+ */
+export default class PostPreview extends Component {
+  view() {
+    const post = this.props.post;
+    const user = post.user();
+    const excerpt = highlight(post.contentPlain(), this.props.highlight, 200);
+
+    return (
+      <a className="post-preview" href={app.route.post(post)} config={m.route} onclick={this.props.onclick}>
+        <span className="post-preview-content">
+          {avatar(user)}
+          {username(user)}
+          {humanTime(post.time())}
+          <span className="excerpt">{excerpt}</span>
+        </span>
+      </a>
+    );
+  }
+}