Add modified_id property to groups

Track hours on this issue to: http://dev.nysenate.gov/issues/5684

1. Add modified_id column to civicrm_group table (to store civicrm_contact.id)
2. Set modified_id value when group is updated
3. Add 'Modified By' readonly display field to Group Settings form when action = update (below Created By).

Note that currently we are updating created_id during update mode, we should fix that. This field is never updated.