Adds alt text to the CC image on public syllabus pages

fixes CNVS-14604

Test Plan:
  - Make a course publicly available.
  - Set it to have a Creative Commons license
  - Go to the Syllabus page
  - The Alt Text of the CC image at the bottom should be
    more descriptive of the license something like
    "CC Attribution" rather than "CC_by"

Change-Id: I09cfa277283f301c63775e05de577632657aeb31
Reviewed-on: https://gerrit.instructure.com/43269
Reviewed-by: Dan Minkevitch <dan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>