Adds "Draw an Answer" to student exercises

Adds a "Draw an Answer" option to student exercise work.  All the other features of free responses still apply here (sorting, canceling editing, etc).   

Thanks to @Dantemss for his great work on the sketchily gem to make this PR pretty straightforward.

@kevinburleigh75 should do a code review and @kjdav should test it out before we merge.