Allow admins to make specific project services unavailable
Description
An administrator may wish to disable the use of certain project services globally, or for a particular group or project. In particular, the "terminal websocket" support in gitlab-ce!7690 may need disabling in some contexts.
Proposal
Extend service templates so the administrator can make some of them unavailable to project owners.
Stretch: Allow their use to be restricted to a whitelist or blacklist of names (by user, group or project).