Allow an author-specified background image for ImageQuestions

For [#59358426] (drawing questions with author-specified background images) to work, we take the specified background image URL and copy it to the image_url of the answer (which would contain the URL of the snapshot image in a Shutterbug question, and would be blank for other drawing questions).

https://www.pivotaltracker.com/story/show/59358426
Adds form fields to provide bg image for ImageQuestions

The last of the big pieces for an author-specified background image to annotate. [#59358426]

https://www.pivotaltracker.com/story/show/59358426