astakos: Add user logic

Add a `user_logic` file that serves as a common endpoint for actions on
users. The actions that this file offers act as a wrapper above
`activation_backends` and are used by `snf_manage` and `im` views.

Also, add a `validate_user_action` function in `activation_backends`, in
order to export this user logic to any other module that wants to use
it.
astakos: Add tests for user_logic

Add tests under `im` that verify that the functions in `user_logic.py`
are working as intended.