Add filtered text support to long answer rubrics and answers.

Problem/Motivation

Sometimes it is necessary that a long answer question contain something like an image or rich text. This isn't possible right now as the rubric and user answer are plain text only.

Proposed resolution

Make both of these fields use filtered text support.

Remaining tasks

Add filtered text support for the rubric field.
Add filtered text support for the answer field.
Add plain/filtered toggle for each long answer question.
Set existing formats to plaintext.

User interface changes

Filtered format option when allowed.

API changes

None

Data model changes

New "rubric_format" and "answer_format" columns.