FIX applies pre-commit
IMPL adds test for share of backend user
FIX pre-commit pass
IMPL removes mail.activity.mixin from dummy model because it is not needed for the test
IMPL renames and divide the base_group_backend into 2 groups one that provide the basic rights and another that allow login in the app
IMPL changes backend ui users to a user type
FIX pre-commit pass
FIX removes useless imports
FIX adds share to group_backend_ui_users
IMPL adds mail_channel to access rights
FIX tests now working
FIX pre-commit pass
