add 'download all files' link to ember quiz menu

fixes CNVS-13423

test plan:
  - enable draft state and quiz sttas
  - as a teacher
    - create a quiz with a file upload question
    - view the quiz
    - there is now an option to "download all files" in the settings menu
    - it should initially be disable if there are no submissions yet
    - trying to click on it disabled should close the menu and flash a message
      telling why it is disabled

  - as a student
    - take the quiz, and upload a file

  - as a teacher
    - view the quiz again
    - the "download all files" should now be enabled
    - click and make sure files download successfully

Change-Id: Ia9455b18916ef6fcb403b742671104fdcd17fd39
Reviewed-on: https://gerrit.instructure.com/35952
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>