Allow group administrators to bypass PathRequestSubscriber on admin routes

Problem/Motivation

When administering group domains, if one configures a domain that is also used for administration on a particular group, it becomes impossible to edit other groups. That's because PathRequestSubscriber does not check to see if the requested route is an admin route.

I noticed that this does not happen if a "main" domain is configured, so perhaps that's by design? If so, I think the main domain property should be a required value.

If not, I think PathRequestSubscriber could check if the route object has the _admin_route option set to TRUE and all group administrators to bypass the NotFoundException