FIX: various fixes to draft system

- destroyDraft which is called when we cancel a draft is now async,
  removing race conditions when you click "reply" to a post and are
  already editing. We used to trigger double dialogs for cancelling
  drafts which was confusing.

- Remove reply as new topic / reply as pm keys, they are no longer
  used and only caused confustion. For example we used to pop up a
  warning when you are composing a reply and flick to reply as
  new topic

- Remove createTopic key, this was a bug that proliferated. Whenever
  creating a topic via the C shortcut or clicking on new topic on full
  screen search the correct new topic draft key will be used
  consistently

- When abandoning an edit we now say "Are you sure you want to discard
  your changes" (instead of abandon your post which is confusing)

app/assets/javascripts/select-kit/components/composer-actions.js.es6

@@ -5,8 +5,7 @@ import {
   CREATE_TOPIC,
   CREATE_SHARED_DRAFT,
   REPLY,
-  EDIT,
-  NEW_PRIVATE_MESSAGE_KEY
+  EDIT
 } from "discourse/models/composer";
 
 // Component can get destroyed and lose state
@@ -209,7 +208,6 @@ export default DropdownSelectBoxComponent.extend({
 
   _replyFromExisting(options, post, topic) {
     this.closeComposer();
-
     this.openComposer(options, post, topic);
   },
 
@@ -244,6 +242,7 @@ export default DropdownSelectBoxComponent.extend({
     options.action = CREATE_TOPIC;
     options.categoryId = this.get("composerModel.topic.category.id");
     options.disableScopedCategory = true;
+    options.skipDraftCheck = true;
     this._replyFromExisting(options, _postSnapshot, _topicSnapshot);
   },
 
@@ -269,7 +268,7 @@ export default DropdownSelectBoxComponent.extend({
     options.action = PRIVATE_MESSAGE;
     options.usernames = usernames;
     options.archetypeId = "private_message";
-    options.draftKey = NEW_PRIVATE_MESSAGE_KEY;
+    options.skipDraftCheck = true;
 
     this._replyFromExisting(options, _postSnapshot, _topicSnapshot);
   },