Add posts app using generic document

This allows posts to be listed at /posts in the UI.
Rename post partial used in organization view

The view post view is called post/view.html by default. The partial in
the organization view/form had a conflicting name, so I've renamed that
so we can use the default name for the actual post view.
Add page to view individual post

This shows the post label, role and area name.

Will also need to add the person/organization that these are attached to
if relevant.