Improve extensions page

- Allow extensions to define an icon in their flarum.json
- Show a "please wait" modal when enabling/disabling an extension
- Styling tweaks

js/admin/src/components/LoadingModal.js

@@ -0,0 +1,19 @@
+import Modal from 'flarum/components/Modal';
+
+export default class LoadingModal extends Modal {
+  isDismissible() {
+    return false;
+  }
+
+  className() {
+    return 'LoadingModal Modal--small';
+  }
+
+  title() {
+    return 'Please Wait...';
+  }
+
+  content() {
+    return '';
+  }
+}