Remove inheritance of MembershipConfig form from MembershipStatus form.

Remove inheritance of MembershipConfig form from MembershipStatus form.

This is part of trying to move towards an entity form (ie. support custom data) for MembershipType.

Currently the 2 forms share a parent class - but really the add buttons is the only shared functionality.

This change uses the EntityTrait to add the buttons as well. There is some minor tidy up in there
but I tried to keep it very limited.

I switched only one field over to being added via addField, the parent mechanism so far