Adds helper functions to display entityref parents

[aaronschachter]

Fixes last item left in #606

• Adds helper functions which query entityreference fields to see where a given node is referenced.
• Moves hook_node_access check into dosomething_user to have one place to deny access to view content types that only staff should have view access to (facts, images).
• Displays three sections on a Fact node (which is an internal view only staff can see):
  • parent nodes which reference the Fact via field_facts (this will include campaigns and fact_page)
  • parent nodes which reference the Fact via field_fact_problem
  • parent nodes which reference the Fact via field_fact_solution
• Similarly displays parent nodes which reference a given image node on the image node's node_view (which again, only staff can see

[angaither]

this would be a good place to add caching in a later lifetime.

[angaither]

👍