bug 839559: Enable section editing

* Leave section editing disabled for non-en-US locales, for now.

* Switch to injecting section editing buttons on the client side, rather
  than on the server

* Use KumaScript preview POST to update the section after a save, to
  avoid a race condition with the deferred rendering system

* Render macros on section edit response

* Ensure https:// links on redirect in Vagrant