Add scopes to group API resources

[lyzadanger]

Needed for #865

As part of the group-selector changes and community groups work, we'll be returning groups from the API that a user is a member of, but that may or may not be "active" on a current document URI based on their scope restrictions.

We need to be able to give the client hints about what the valid scopes for certain groups are so that it can a) determine whether a current group is in scope or not and possibly b) render information about scope to users (I'm glossing over some nuance and UX here).

It may make sense to use the existing pattern of an expand parameter for this (allow clients to determine how much granularity they get back for scope information).