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/ComposerBody.js

@@ -0,0 +1,107 @@
+import Component from 'flarum/Component';
+import LoadingIndicator from 'flarum/components/LoadingIndicator';
+import TextEditor from 'flarum/components/TextEditor';
+import avatar from 'flarum/helpers/avatar';
+import listItems from 'flarum/helpers/listItems';
+import ItemList from 'flarum/utils/ItemList';
+
+/**
+ * The `ComposerBody` component handles the body, or the content, of the
+ * composer. Subclasses should implement the `onsubmit` method and override
+ * `headerTimes`.
+ *
+ * ### Props
+ *
+ * - `originalContent`
+ * - `submitLabel`
+ * - `placeholder`
+ * - `user`
+ * - `confirmExit`
+ * - `disabled`
+ *
+ * @abstract
+ */
+export default class ComposerBody extends Component {
+  constructor(props) {
+    super(props);
+
+    /**
+     * Whether or not the component is loading.
+     *
+     * @type {Boolean}
+     */
+    this.loading = false;
+
+    /**
+     * The content of the text editor.
+     *
+     * @type {Function}
+     */
+    this.content = m.prop(this.props.originalContent);
+
+    /**
+     * The text editor component instance.
+     *
+     * @type {TextEditor}
+     */
+    this.editor = new TextEditor({
+      submitLabel: this.props.submitLabel,
+      placeholder: this.props.placeholder,
+      onchange: this.content,
+      onsubmit: this.onsubmit.bind(this),
+      value: this.content()
+    });
+  }
+
+  view() {
+    // If the component is loading, we should disable the text editor.
+    this.editor.props.disabled = this.loading;
+
+    return (
+      <div>
+        {avatar(this.props.user, {className: 'composer-avatar'})}
+        <div className="composer-body">
+          <ul className="composer-header">{listItems(this.headerItems().toArray())}</ul>
+          <div className="composer-editor">{this.editor.render()}</div>
+        </div>
+        {LoadingIndicator.component({className: 'composer-loading' + (this.loading ? ' active' : '')})}
+      </div>
+    );
+  }
+
+  /**
+   * Draw focus to the text editor.
+   */
+  focus() {
+    this.$(':input:enabled:visible:first').focus();
+  }
+
+  /**
+   * Check if there is any unsaved data – if there is, return a confirmation
+   * message to prompt the user with.
+   *
+   * @return {String}
+   */
+  preventExit() {
+    const content = this.content();
+
+    return content && content !== this.props.originalContent && this.props.confirmExit;
+  }
+
+  /**
+   * Build an item list for the composer's header.
+   *
+   * @return {ItemList}
+   */
+  headerItems() {
+    return new ItemList();
+  }
+
+  /**
+   * Handle the submit event of the text editor.
+   *
+   * @abstract
+   */
+  onsubmit() {
+  }
+}