Support for expanded ImageQuestions in LARA

LARA's ImageQuestions have two prompt fields ("prompt" and "drawing_prompt": the latter shows up over the drawing canvas, the former underneath, next to an optional open-response textarea), and both of them are optional (although a question with both of them blank would be confusing). These changes remove the validation that enforced length on ImageQuestion prompt attributes to allow for the LARA questions to be published to the portal. It also adds support for the drawing_prompt attribute and adds it to the teacher reports and learner reports. Researcher reports get a test validating that they will still run if the prompt provided for the head of that report is blank or nil.

This refers to PT stories https://www.pivotaltracker.com/story/show/59636810 and https://www.pivotaltracker.com/story/show/58968402 .