make rubric description dialog preserve line breaks when displaying HTML

Originally we were simply extracting the text from the relevant textarea
and just dumping it into the html via $.fn.FillTemplateData,
the problem with that method is that it doesn't preserve the line breaks
the teacher entered into the field. The first attempt at fixing the
problem was to simply insert <br> tags and tell fillTemplateData that
we're always sending HTML data, unfortunately that opens up some
potential for XSS so we're escaping the content before messing with the
line breaks, and then shipping it off as HTML.

fixes CNVS-9748

Test Plan:
- as a teacher create an assignment
- add a rubric and include line breaks in the long description
- the dialog displaying the long description should have line breaks
  when not editing

Change-Id: I839bdde88ccd35bb55750ec63054d7c606ef05b6
Reviewed-on: https://gerrit.instructure.com/34472
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>