auth: explicit user permission should not blindly overrule permissions through user groups

Before, explicit permissions of a user could shadow higher permissions that
would otherwise be obtained through a group the user is member of.
That was confusing and fragile: *removing* a permission could then suddenly
give a user *more* permissions.

Instead, change the flag for controlling internal permission computation to
*not* use "explicit". Permissions will then add up, no matter if they are
explicit or through groups.

The change in auth.py is small, but read the body of __get_perms to see the
actual impact ... and also the clean-up changeset that will come next.

This might in some cases be a behaviour change and give users more access ...
but it will probably only give the user that was intended. This change can thus
be seen as a bugfix.

Some tests assumed the old behaviour. Not for good reasons, but just because
that is how they were written. These tests are updated to expect the new
behaviour, and it has been reviewed that it makes sense.

Note that this 'explicit' flag mostly is for repo permissions and independent
of the 'user_inherit_default_permissions' that just was removed and is about
global permissions.