use organization_id consistently

Sometimes we have organization_id, sometimes id. Consolidate these.
Remove unneeded :name attribute

It’s never used.
Add 'MP' role to party memberships

The memberships are specifically of them being MP, rather than simply
party members or any other positions in the party (which should have
separate entries). This will also be easier to search/filter on. It
also allows us to simplify the code, as all memberships will have the
same fields.
Refactor Membership classes

No longer need so much special case code.