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.
Link to the post from membership listing
Add icon-posts.gif

This is the icon that appears on the homepage of the instance, linking
through to the list of all posts in an instance.
Populate post's organization for each membership

When a person's membership with a post has no organization on the
membership we instead use the posts's organization directly. This
ensures that the posts's organization is populated before we get to the
template.