Group master can manage group members

In the current implementation only Group Owner can manage users within a group.

This change will grand Group Master to manage users within a group. It will also prevent a master to add a user with a higher access role then it's own (Master cannot create Owner, only Owner can create Owner).

Use case for this change is a distributed teams where you have localized managers who are responsible for group setup but should not be able to do operations like deletion of repositories.