"active" flag in Crowd should be set to false once a user is no longer a member of any groups

Users that don't have the active column set to F in Crowd's database will still show in Confluence's people directory and at-mentions even if they're not members of any groups.

User Management should set the active flag to false when a user is no longer a member of any groups (and conversely, should set it to true once they're re-added to groups).