[bug 935068] Implemented "Articles that link to an image".

This adds a hook into the parser to save a connection between
documents and images. Then it displays the documents on the
image details page.

It was pretty easy to do by following the patterns @mythmon
did for document links.

To verify:

* Run the test suite
* Test manually:

1. Upload an image to the gallery.
2. Include the image in a document by editing or creating a new document and approving the revision.
3. Go to the Image details page and verify that the document is listed.

r?