Issue 582: Tenant and Global admins should be able to list the members of a group.
583 - Restoring a revision should create a restore activity event.

The oae:activityType will be set to 'content-restored-revision' when
an older revision gets restored.
Merge pull request #583 from simong/issue-583-restored-revision-activity

Should have an activity for restoring a revision
Merge pull request #582 from simong/issue-582-group-members-admins

Global or tenant admins should be able to see all group members.