FEATURE: Shared Drafts

This feature can be enabled by choosing a destination for the
`shared drafts category` site setting.

* Staff members can create shared drafts, choosing a destination
category for the topic when it is published.

* Shared Drafts can be viewed in their category, or above the
topic list for the destination category where it will end up.

* When the shared draft is ready, it can be published to the
appropriate category by clicking a button on the topic view.

* When published, Drafts change their timestamps to the current
time, and any edits to the original post are removed.

lib/topic_query.rb

@@ -44,7 +44,8 @@ class TopicQuery
          per_page
          visible
          guardian
-         no_definitions)
+         no_definitions
+         destination_category_id)
   end
 
   # Maps `order` to a columns in `topics`
@@ -606,6 +607,11 @@ class TopicQuery
       result = apply_ordering(result, options)
       result = result.listable_topics.includes(:category)
 
+      if options[:destination_category_id]
+        destination_category_id = get_category_id(options[:destination_category_id])
+        result = result.includes(:shared_draft).where("shared_drafts.category_id" => destination_category_id)
+      end
+
       if options[:exclude_category_ids] && options[:exclude_category_ids].is_a?(Array) && options[:exclude_category_ids].size > 0
         result = result.where("categories.id NOT IN (?)", options[:exclude_category_ids]).references(:categories)
       end