Check group id existence in storage implemented

Issue #29
I think a proper way to fix the problem described in https://github.com/C3Junior/c3-web/issues/29 is to change the way how we map groups to C3:
instead of using natural group ids (primary keys in local DB) we could introduce a UUID unique key as a new group field.
then simply map group to C3 by this UUIDs....

Solution in this PR will work, but it looks to me like a duct tape. :(
The initial decision to use group ids for mapping (which I'm fully responsible for :) was very bad.