auth: always consider the repo group owner an admin when computing it's permissions

When computing repo group permissions in repository_group_permissions(), always
give admin permissions to the group owner.

That is similar to how repository_permissions() gives admin permissions to the
repo owner.

The extra computation shouldn't cause any extra database hits or make the
computation more complex or expensive, so that should be fine for stable.

Note: This will leave behind some (automaticly added) explicit permissions. I
consider this a very minor glitch, not worth addressing.