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/admin/ExtensionsPage.less

@@ -22,6 +22,10 @@
 .ExtensionListItem.disabled .ExtensionListItem-content {
   opacity: 0.5;
   color: @muted-color;
+
+  .ExtensionListItem-icon {
+    opacity: 0.5;
+  }
 }
 .ExtensionListItem-controls {
   float: right;
@@ -41,6 +45,9 @@
 .ExtensionListItem-icon {
   float: left;
   margin-left: -42px;
+  text-align: center;
+  padding: 3px;
+  font-size: 14px;
 }
 .ExtensionListItem-title {
   font-size: 14px;