Accept rich text and formatting in rubric notes

[erdogmush]

If rubric notes are formatted, yaml parsing fails miserably with no meaningful error reporting and the grader mysteriously produces no report. This is a pain since it's hard to tell if a rubric note includes any rich text, and it's almost impossible to remove any formatting short of deleting the rubric and adding it again. So make sure that formatting text is accepted via a pre-processing step that removes formatting/rich-text from a rubric note and coverts to plain text first, and then parses it.

[colelloa]

i had a difficult time replicating this issue