Quiz stats: escape HTML in answers in SA CSV

Closes CNVS-9167

TEST PLAN
---- ----

  - create a quiz with the question types listed in the ticket
  - take the quiz as a student and fill in answers like these:
    + <em>fill in the blank</em>
    + <em>fill in multiple blanks</em>
    + ...
  - visit statistics page and generate student analysis report CSV
  - open the sheet and verify the answers do not include the <em />
    tags, only the text within

Change-Id: I1523578801e27beeb4b29394fdcba93fdbda9bd4
Reviewed-on: https://gerrit.instructure.com/41234
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>