Grant 'Manage group members' permission as a custom ability

Description

A maintainer of a group cannot manage that group members.

So to manage group members, one have to be owner, but being owner makes him capable of deleting/altering that group... Which is not wanted in some use-case : for example delegating membership management to a group leader user, without giving him permission to delete/edit the group.

Proposal

Create 'Manage group members' as a customizable permission to be able to be added to any base role

Links / references

https://forum.gitlab.com/t/why-groups-master-cant-manage-members/6280

https://docs.gitlab.com/ce/user/permissions.html

Availability & Testing

Write tests that ensure any role with "Manage group members" consumes a seat.