bug 839559: Enable section editing

DO NOT MERGE (yet) - my eyes are totally glazing over on this one, and I'd like another fresh pair to take a look. I might have to take a break from this for a bit. 

I did find a few more rough edges to section editing, and I think this fixes them:

* Enable section editing for translated pages.

* Switch to injecting section editing buttons on the client side, rather
  than on the server, to fix some ID mismatch issues

* Use KumaScript preview POST to update the section after a save, to
  avoid a race condition with the deferred rendering system

* Ensure https:// links on redirect in Vagrant