[fix bug 1042552] WIP Display membership filter only when the group is accepting new members by request

@glogiotatidis this solution solves the problem in the quickest way but I think it is not safe nor scalable because:

If the group is not accepting members by request and you force the url param filtr (/group/creative/?filtr=pending_members), the filtering will still happen on the backend and only members with pending status will be returned (probably 0) when ishould be returned all the members.
eventually we may need to add more filters to the MembershipFilterForm and we need to go for the approach with dynamic fields at form level.
so, do you think this should be implemented as dynamic fields at form level?
I would like to get your feedback on this before go ahead, thx!