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

less/admin/DashboardPage.less

@@ -1,18 +1,11 @@
 .DashboardPage {
-  background: @control-bg;
+  background: @body-bg;
   color: @control-color;
   min-height: 100vh;
-
-  @media @desktop-up {
-    .container {
-      padding: 30px;
-      margin: 0;
-    }
-  }
 }
 
-.DashboardWidget {
-  background: @body-bg;
+.Widget {
+  background: @control-bg;
   color: @text-color;
   border-radius: @border-radius;
   padding: 20px;