Adds bg_source attribute to ImageQuestions

Using this to distinguish between Snapshot questions and blank-background Draw questions for [#58968010]. Using a string as the type so we can eventually allow users to specify for [#59358426].

https://www.pivotaltracker.com/story/show/58968010
https://www.pivotaltracker.com/story/show/59358426
Authors can select Shutterbug or Drawing; buttons

There's a select menu for picking if an ImageQuestion is from Shutterbug or from a Drawing, and the views render different buttons depending on the selection.
Open the image question window without snapshot

[#58968010] Opening the image question modal without first taking a DOM snapshot actually gets us all the behavior we want, with a few details still needed to account for this new use case.

https://www.pivotaltracker.com/story/show/58968010
Hide the "replace snapshot" if it isn't a snapshot

[#58968010]
Set buttons to edit existing drawing answer

[#58968010] Checking the thumbnail as well as the answer source to show us if there's an existing answer, and thus set the buttons properly. (Also, hide the "draw" button if we're showing "edit".)

https://www.pivotaltracker.com/story/show/58968010
Merge branch 'image-question'