Add ability to sync user with a case

This is the first part of the features needed for call center
functionality.
Close case on user archive/delete
Merge branch 'master' into sync-users-to-cases
Final touches to user/case syncing

- Restrict project config fields to previewer
- Sync custom user data
- Add groups to case owner select
unused import
fix weird django_digest test import dependency issue
Move sync code from couch_user to commcare_user.
Fix failing tests
Merge branch 'master' into sync-users-to-cases
i18n
Remove relics of old changes
Refactor project selection for commcare users

Having `.projects` didn't make sense as commcare users have only one
project..
Use proper defaults for call center settings
Refactored call center enabled into the properties
Refactor post save signal

No longer prevent user from saving if sync fails.
Handle multiple errors from signal
Merge pull request #401 from dimagi/sync-users-to-cases

Sync users to cases