storage: change the type of groups_id

Now a user have a list of groups, and not only one group.
storage: added method for getting users by group
storage: improve behavior of find_user_by_email

Raising UserNotFoundError when the user doesn't exist. I really expected
pymongo to raise an exception here, but since they don't, I'm falling
back to the plain old if not <thing>: raise ThingNotFoundError.

Related to #6.
zabbix: adding users that already exist to group

Also rejecting to adding a user to a group twice.

Related to #6.
storage: remove single quotes

Because I want.
storage: add method to add a user to a group

Related to #6.
storage: add method to remove user from group

Related to #6.
backends/zabbix: also add the user to the group in the storage

Related to #6.