Major CSS revamp

- Get rid of Bootstrap (except we still rely on some JS)
- Use BEM class names
- Rework variables/theme config
- Fix various bugs, including some on mobile

The CSS is still not ideal – it needs to be cleaned up some more. But
that can be a focus for after beta.

js/lib/components/AlertManager.js

@@ -0,0 +1,70 @@
+import Component from 'flarum/Component';
+import Alert from 'flarum/components/Alert';
+
+/**
+ * The `AlertManager` component provides an area in which `Alert` components can
+ * be shown and dismissed.
+ */
+export default class AlertManager extends Component {
+  constructor(...args) {
+    super(...args);
+
+    /**
+     * An array of Alert components which are currently showing.
+     *
+     * @type {Alert[]}
+     * @protected
+     */
+    this.components = [];
+  }
+
+  view() {
+    return (
+      <div className="AlertManager">
+        {this.components.map(component => <div className="AlertManager-alert">{component}</div>)}
+      </div>
+    );
+  }
+
+  /**
+   * Show an Alert in the alerts area.
+   *
+   * @param {Alert} component
+   * @public
+   */
+  show(component) {
+    if (!(component instanceof Alert)) {
+      throw new Error('The AlertManager component can only show Alert components');
+    }
+
+    component.props.ondismiss = this.dismiss.bind(this, component);
+
+    this.components.push(component);
+    m.redraw();
+  }
+
+  /**
+   * Dismiss an alert.
+   *
+   * @param {Alert} component
+   * @public
+   */
+  dismiss(component) {
+    const index = this.components.indexOf(component);
+
+    if (index !== -1) {
+      this.components.splice(index, 1);
+      m.redraw();
+    }
+  }
+
+  /**
+   * Clear all alerts.
+   *
+   * @public
+   */
+  clear() {
+    this.components = [];
+    m.redraw();
+  }
+}