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.
Can you please fix this point?