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

framework/core/less/lib/App.less

@@ -3,6 +3,7 @@
   padding-top: @header-height;
   padding-bottom: 50px;
   overflow-x: hidden;
+  min-height: 100vh;
 
   @media @phone {
     padding-top: @header-height-phone;