use new case processing in 5% of the domains

@dannyroberts should have context for this.

Also adds a new type of toggle that can do deterministic randomness

http://manage.dimagi.com/default.asp?146788

Also adds an admin-only UI for viewing the enabled toggles for a domain.
![image](https://cloud.githubusercontent.com/assets/66555/5050060/ec6279a6-6bf8-11e4-9a04-85c4ddb1b5f6.png)

And minor tweaks to edit toggle UIs for something that is randomly set.

![image](https://cloud.githubusercontent.com/assets/66555/5050078/05848410-6bf9-11e4-9056-6ac8d38e8f56.png)

The only non-admin-facing change is: https://github.com/dimagi/commcare-hq/commit/d4ce65280c8d56f58bdc24b8be116306cfc6421c

I assume this works, but is a bit confusing and possibly (?) easy to test. I think the idea is md5 returns 128 bits and that `float.fromhex(foo)` always returns an integral value (i.e xxxx.0), so it's the same as `float(int.fromhex(foo))`, so dividing by the largest int available in 128 bits, we get a number between 0 and 1. Worth asserting that? Or soft-asserting it with a log.error? And maybe a docstring.