Remove prompt length validation for ImageQuestions.

The allows ImageQuestions from LARA, where this field is optional, to be reliably published to the portal. See [#58968402].

https://www.pivotaltracker.com/story/show/58968402
MIGRATION: Adds drawing_prompt to ImageQuestion.

[#59636810] Just adding an attribute and column to Embeddable::ImageQuestion.

https://www.pivotaltracker.com/story/show/59636810
Set up the runtime API to accept drawing_prompts.

[#59636810] Now ImageQuestions should be properly storing and updating drawing_prompts in embeddables published from LARA.

https://www.pivotaltracker.com/story/show/59636810
Show drawing_prompts for ImageQuestions in reports.

[#59636810] This should handle teacher and learner reports, putting the drawing_prompt in just above the existing prompt with a <br /> to break between them.

https://www.pivotaltracker.com/story/show/59636810
Spec test to verify researcher reports with ImageQuestion.

[#58968402] This ensures a nil prompt for an ImageQuestion won't blow up researcher reports, now that we're allowing empty prompts. 

https://www.pivotaltracker.com/story/show/58968402
Merge pull request #45 from concord-consortium/image-questions

Support for expanded ImageQuestions in LARA.

Looks good, specs pass.