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/lib/components/Button.js

@@ -0,0 +1,55 @@
+import Component from 'flarum/Component';
+import icon from 'flarum/helpers/icon';
+import extract from 'flarum/utils/extract';
+
+/**
+ * The `Button` component defines an element which, when clicked, performs an
+ * action. The button may have the following special props:
+ *
+ * - `icon` The name of the icon class. If specified, the button will be given a
+ *   'has-icon' class name.
+ * - `disabled` Whether or not the button is disabled. If truthy, the button
+ *   will be given a 'disabled' class name, and any `onclick` handler will be
+ *   removed.
+ *
+ * All other props will be assigned as attributes on the button element.
+ *
+ * Note that a Button has no default class names. This is because a Button can
+ * be used to represent any generic clickable control, like a menu item.
+ */
+export default class Button extends Component {
+  view() {
+    const attrs = Object.assign({}, this.props);
+
+    delete attrs.children;
+
+    attrs.className = (attrs.className || '');
+    attrs.href = attrs.href || 'javascript:;';
+
+    const iconName = extract(attrs, 'icon');
+    if (iconName) attrs.className += ' has-icon';
+
+    const disabled = extract(attrs, 'disabled');
+    if (disabled) {
+      attrs.className += ' disabled';
+      delete attrs.onclick;
+    }
+
+    return <a {...attrs}>{this.getButtonContent()}</a>;
+  }
+
+  /**
+   * Get the template for the button's content.
+   *
+   * @return {*}
+   * @protected
+   */
+  getButtonContent() {
+    const iconName = this.props.icon;
+
+    return [
+      iconName ? icon(iconName) : '',
+      <span className="label">{this.props.children}</span>
+    ];
+  }
+}