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/WelcomeHero.js

@@ -0,0 +1,46 @@
+import Component from 'flarum/Component';
+import icon from 'flarum/helpers/icon';
+
+/**
+ * The `WelcomeHero` component displays a hero that welcomes the user to the
+ * forum.
+ */
+export default class WelcomeHero extends Component {
+  constructor(...args) {
+    super(...args);
+
+    this.hidden = localStorage.getItem('welcomeHidden');
+  }
+
+  view() {
+    if (this.hidden) return <div/>;
+
+    const slideUp = () => {
+      this.$().slideUp(this.hide.bind(this));
+    };
+
+    return (
+      <header className="hero welcome-hero">
+        <div class="container">
+          <button className="close btn btn-icon btn-link" onclick={slideUp}>
+            {icon('times')}
+          </button>
+
+          <div className="container-narrow">
+            <h2>{app.forum.attribute('welcomeTitle')}</h2>
+            <div className="subtitle">{m.trust(app.forum.attribute('welcomeMessage'))}</div>
+          </div>
+        </div>
+      </header>
+    );
+  }
+
+  /**
+   * Hide the welcome hero.
+   */
+  hide() {
+    localStorage.setItem('welcomeHidden', 'true');
+
+    this.hidden = true;
+  }
+}