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/PermissionsPage.less

@@ -1,6 +1,11 @@
 .PermissionsPage-groups {
   background: @control-bg;
-  padding: 20px 0;
+  border-radius: @border-radius;
+  max-width: calc(~'100% - 60px');
+  display: block;
+  margin-left: 30px;
+  overflow-x: auto;
+  padding: 8px 0 8px;
 }
 .Group {
   width: 90px;