Fix a bug where MP group memberships wouldn't be created
Fix a bug where MP group memberships wouldn't be created

Since we were doing get_or_create without taking `since`
into account, only the first MP group membership with the same (group,
role) pair would be created. Including since in get_or_create filter
fixes this problem.

ManoSeimas/manoseimas.lt,ManoSeimas/manoseimas.lt,ManoSeimas/manoseimas.lt,ManoSeimas/manoseimas.lt