Remove skip memberships flag

Remove the `skipMemberships` flag, which was causing problems because it set the flag at the schema level and could therefore cause race conditions across requests. I've replaced it with two methods, `findMemberships` and `populateMemberships` which have to be explicitly called in order to take effect. I've also had to add `populateMemberships` to the `Membership` schema as a stub implementation so that the app.js file can remain generic and support all object types.

Fixes https://github.com/mysociety/popit/issues/631
Fixes https://github.com/mysociety/popit-api/issues/77