Move membership finding into generic document app

Rather than the person, organization and post apps each having their own
membership finding code I've refactored things so that the generic
document app is responsible for finding memberships.
Add list of memberships to post view page

This lists all the people who are associated with a post via a
membership.

Currently this just passes the post's organization through to the
membership template, but there may be something smarter/different we can
do to clean up the display.