Make zone and org deletions use AJAX requests

This involves a bunch of new steps:

* Moving `delete_zone` code to `central` app.
* Migrating code to use `doRequest`
* Setting up a way for the front-end to receive the type of message it needs (`JsonResponseMessage` if error, `django.contrib.messages` message if succeeding, when page reloads)

Testing:

* Did manual testing on the central server `zone` and `org` deletion, works like a champ!
* Need testing on the `central` and `distributed` side for `facility` deletion and users moving between groups.

@rtibbles do you have a min to review?  I'm most interested in what you think of the design for 67a0c0c .  For me, it's nice because it's a localized way to do front-end/back-end interaction (decorator) that can easily be removed if and when we move to a backbone / page-doesn't-need-to-reload method for doing these same things.