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/AdminHeader.less

@@ -0,0 +1,19 @@
+.AdminHeader {
+  background: @control-bg;
+  margin-bottom: 20px;
+  padding: 20px 0;
+
+  h2 {
+    margin-top: 0;
+    margin-bottom: 10px;
+    color: @muted-color;
+  }
+
+  .AdminHeader-description {
+    margin: 0;
+  }
+
+  .icon {
+    margin-right: 15px;
+  }
+}