Add a toolbar area to the TextEditor component

This is to be used for formatting buttons and the like. Although it is
displayed in the controls area at the moment, it is conceptually
different and may be relocated to the top of the text editor in the
future.

less/forum/Composer.less

@@ -353,6 +353,11 @@
     margin-right: 10px;
   }
 }
+.TextEditor-toolbar {
+  .Button--icon {
+    width: 28px;
+  }
+}
 
 @media @tablet-up {
   .TextEditor-controls {