Make memberships work with coalition style member property

If a membership uses a member object with '@type' and 'id' fields then
populate that before trying to display the membership listing. We then
use that object first in the view if available, then fallback to the old
way if there isn't one.
handle instance not unique error on instance creation

check in template for slug_not_unique error code and if present display
an error and a link to the existing instance of that name.
Handle new-style memberships in templates

When an organization is related to an organization or a person related
to a person using the member property of a relationship this ensures
that relationship is displayed correctly.
Make sure that the membership is valid before rendering

This prevents 500 errors when certain pieces of information about a
membership are missing or they refer to records that no longer exist.
Merge branch 'fix-coalition-display'
merge latest changes from master into 468-person-page-again