Remove dekicompat backend, usage of it, and lingering MindTouch API refs

Don't merge this yet.

As part of trying to track down [bug 813240](https://bugzilla.mozilla.org/show_bug.cgi?id=813240), I noticed that a lot of the most-suspect-looking code paths involved our old deki/MindTouch backend and associated code. In the interest of not having to track through all of that stuff to hunt down a bug, I decided to give it the axe.

This will, once merged, get rid of:

* The auth backend for looking up MindTouch users.
* Any code which relied on that backend (which shouldn't be relevant since all our auth is Persona + Django now).
* A bunch of other seemingly-dead code paths found by grepping the kuma source for references to Deki/MindTouch.

This will probably want some review before we even think about merging, to ensure it's not deleting something we still actually use in some weird way. I also expect it's going to need some refinement to get tests passing, but wanted to throw the code out here so we could start looking at it for things that shouldn't be removed.