limit character count of fill in the blank answers

fixes CNVS-1795
It doesn't make sense to allow fill in the blank answers past a certain
length, due to the strict matching we use for grading.  This commit
introduces a validated limit on the client side to limit these to 80
characters.

Test plan
- Create a fill in the blank / multiple fill in the blank question
- Add an answer longer than 80 characters.
- You should be warned that the limit is unacceptable, and the answer
  should be truncated to 80 characters.

Change-Id: Ifef524f3d30d25e70cb6b12d0c89a8dc8effdc2b
Reviewed-on: https://gerrit.instructure.com/33465
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>