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/ComposerButton.js

@@ -0,0 +1,13 @@
+import Button from 'flarum/components/Button';
+
+/**
+ * The `ComposerButton` component displays a button suitable for the composer
+ * controls.
+ */
+export default class ComposerButton extends Button {
+  static initProps(props) {
+    super.initProps(props);
+
+    props.className = props.className || 'btn btn-icon btn-link';
+  }
+}