FIX: In test mode, initializers were modifying classes over and over

This adds a new property, `pluginId` which you can pass to `modifyClass`
which prevent the class from being modified over and over again.

This also includes a fix for polls which was leaking state between tests
which this new functionality exposed.

plugins/discourse-details/assets/javascripts/initializers/apply-details.js.es6

@@ -15,6 +15,7 @@ function initializeDetails(api) {
   });
 
   api.modifyClass("controller:composer", {
+    pluginId: "discourse-details",
     actions: {
       insertDetails() {
         this.toolbarEvent.applySurround(