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'

spec/controllers/topics_controller_spec.rb

@@ -733,6 +733,7 @@ describe TopicsController do
     describe 'when logged in' do
       before do
         @topic = Fabricate(:topic, user: log_in)
+        Fabricate(:post, topic: @topic)
       end
 
       describe 'without permission' do
@@ -778,7 +779,7 @@ describe TopicsController do
 
         it "returns errors with invalid categories" do
           Topic.any_instance.expects(:change_category_to_id).returns(false)
-          xhr :put, :update, topic_id: @topic.id, slug: @topic.title
+          xhr :put, :update, topic_id: @topic.id, slug: @topic.title, category_id: -1
           expect(response).not_to be_success
         end