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.

app/models/topic.rb

@@ -134,6 +134,8 @@ class Topic < ActiveRecord::Base
   has_many :tag_users, through: :tags
 
   has_one :top_topic
+  has_one :shared_draft, dependent: :destroy
+
   belongs_to :user
   belongs_to :last_poster, class_name: 'User', foreign_key: :last_post_user_id
   belongs_to :featured_user1, class_name: 'User', foreign_key: :featured_user1_id