Management command/function to refresh permission objects

[atodorov]

Permissions in Django are regular DB objects but they are created on the fly when we add new models. Some permissions are applied by default to the Tester group but this is only done for new installations, usually upon first login.

Also when removing objects Django doesn't remove stale permissions from the database but happily introduces the new ones.

Because all of this happens dynamically trying to update these permissions in a data migration doesn't usually work.

We should probably provide a management command that refreshes all permissions in the DB and takes care to correctly assign the defaults for the Tester group.

For reference see #881.

[atodorov]

@schwarzkrieger see django.contrib.auth.management.create_permissions()

also tcms.utils.permissions