Implement static page content section reassignment -- NO FOR SPRINT 30

* Provide the machinery to enable reassignment of the content_section
  while editing a static_page. Care is taken to ensure that when moving
  a ContentGroup parent, it's children move with it.
  (addresses #1819)

This is the critical half of #1819 needed for DB class launch; when this is accepted, that ticket can become a P2 for sprint 30.