Coalitions

Add support for modelling coalitions using a `member` property on memberships which acts as a polymorphic reference to a person OR an organization.

There was some discussion in https://github.com/mysociety/popit-api/commit/9d9f0582ce0d70936ecbac466886e40c9c031ffd about the best way to represent the coalitions that a person is in, but we've decided to revert the main behaviour that commit introduces for the time being and come back to it once we've solved https://github.com/mysociety/popit/issues/306 and https://github.com/mysociety/popit/issues/307.

Note: MongoDB needs to be upgraded on ajax before this can be deployed.

Closes https://github.com/mysociety/popit/issues/324