add correct classes for object attaching on org page

add posts and memberships classes to the lists that contain these in
order allow the ids of the related objects to be correctly attached.
Without this the edit button doesn't act on the model and you end up
creating a new model and the delete button doesn't do anything

Part of #406
call destroy directly on posts model for deleting

Becuase posts are a separate model and not nested within the
organization we need to destroy them directly rather than removing them
from the nested collection and then rely on that cascading the save up
to the organization model. If we don't do this then no API call is made
and the post isn't deleted on the server.

Fixes #406