Display memberships associated with posts

On person pages make sure validMembership returns true for memberships
with a post_id property and relatedObject returns the post's
organization for the membership.
Make validMembership slightly easier to follow

Rather than one long conditional using && and || operators I've split it
up to use if statements. This hopefully makes it a bit easier to follow
what's going on.
Merge branch '654-display-post-memberships' into chris-dev