Fix plugin selectors after angular upgrade

The recent angular update changed the format of the hash keys.
They're now in the format 'object:123' (etc), which isn't a valid
CSS class name any more, but we used the $$hashKey as such.

I used this opportunity to introduce a bit of abstraction there as well

directives/plugin.html

@@ -4,7 +4,7 @@
     Hide {{ ::plugin.name }}
   </button>
 
-  <div ng-bind-html="displayedContent" class="embed" ng-class="::('embed_' + plugin.$$hashKey)"></div>
+  <div ng-bind-html="displayedContent" class="embed" ng-class="::plugin.className"></div>
 </div>
 
 <div ng-hide="plugin.visible">