add created & mod timestamps to users & groups

Fresh installs should work out of the box.
Migration instructions for existing installs:
db.users.update_many({}, {'$set': {'created': datetime.datetime.utcfromtimestamp(0), 'modified': datetime.datetime.utcnow()}})
db.groups.update_many({}, {'$set': {'created': datetime.datetime.utcfromtimestamp(0), 'modified': datetime.datetime.utcnow()}})