remove dead code

Co-dependent on https://github.com/dimagi/commcare-hq/pull/91

remove old django_user_registration library that was a slightly modified version of a common library that seemed like it was in use but actually wasn't.  I exhaustively checked the lack of any actual dependencies on code I removed, but I sanity checked this by registering and activating a new user and then inviting another user, with no problems.

Thereby removing the lib/ directory

remove ancient tests/ directory

remove unused docs app