AdminUX Overhaul (#2409)

- Extensions now have their own pages
- The API for extensions to register permissions and settings has been overhauled via the `flarum/admin/utils/ExtensionData` util
- An extension grid has been added as a widget to the Dashboard page

js/src/admin/components/AppearancePage.js

@@ -7,6 +7,7 @@ import EditCustomHeaderModal from './EditCustomHeaderModal';
 import EditCustomFooterModal from './EditCustomFooterModal';
 import UploadImageButton from './UploadImageButton';
 import saveSettings from '../utils/saveSettings';
+import AdminHeader from './AdminHeader';
 
 export default class AppearancePage extends Page {
   oninit(vnode) {
@@ -21,6 +22,13 @@ export default class AppearancePage extends Page {
   view() {
     return (
       <div className="AppearancePage">
+        <AdminHeader
+          icon="fas fa-paint-brush"
+          description={app.translator.trans('core.admin.appearance.description')}
+          className="AppearancePage-header"
+        >
+          {app.translator.trans('core.admin.appearance.title')}
+        </AdminHeader>
         <div className="container">
           <form onsubmit={this.onsubmit.bind(this)}>
             <fieldset className="AppearancePage-colors">