LOTS of changes to properly handle post/topic revisions

FIX: history revision can now properly be hidden
FIX: PostRevision serializer is now entirely dynamic to properly handle
hidden revisions
FIX: default history modal to "side by side" view on mobile
FIX: properly hiden which revision has been hidden
UX: inline category/user/wiki/post_type changes with the revision
details
FEATURE: new '/posts/:post_id/revisions/latest' endpoint to retrieve
latest revision
UX: do not show the hide/show revision button on mobile (no room for
them)
UX: remove CSS transitions on the buttons in the history modal
FIX: PostRevisor now handles all the changes that might create new
revisions
FIX: PostRevision.ensure_consistency! was wrong due to off by 1
mistake...
refactored topic's callbacks for better readability
extracted 'PostRevisionGuardian'

db/migrate/20141020153415_add_public_version_to_posts.rb

@@ -0,0 +1,15 @@
+class AddPublicVersionToPosts < ActiveRecord::Migration
+  def up
+    add_column :posts, :public_version, :integer, null: false, default: 1
+
+    execute <<-SQL
+      UPDATE posts
+         SET public_version = 1 + (SELECT COUNT(*) FROM post_revisions pr WHERE post_id = posts.id AND pr.hidden = 'f')
+       WHERE public_version <> 1 + (SELECT COUNT(*) FROM post_revisions pr WHERE post_id = posts.id AND pr.hidden = 'f')
+    SQL
+  end
+
+  def down
+    remove_column :posts, :public_version
+  end
+end