FIX: draft not clearing when replying to new topic

This amends our API so we provide it with the draft key when saving a post
this means post creator can clean up the draft consistently even if we are
doing fancy stuff like replying to a new topic or new pm or whatever.

There will be some followup work to clean it up so client never calls destroy
on draft during normal operation and the #create/#update endpoints takes care of it
every time

app/controllers/posts_controller.rb

@@ -671,7 +671,8 @@ class PostsController < ApplicationController
       :auto_track,
       :typing_duration_msecs,
       :composer_open_duration_msecs,
-      :visible
+      :visible,
+      :draft_key
     ]
 
     Post.plugin_permitted_create_params.each do |key, plugin|