Incremental membership editing on /groups/xxx/members

First cut at feature for https://meta.discourse.org/t/improved-group-members-management/16768/14

This is not ready for merge and there are no tests. I'm a novice with the Ember stuff, hopefully I'm not far wrong in the approach. Perhaps @eviltrout could take a look?
we usually use "user_id" here, cause manager_id is confusing.