add all_dates to assignment api for mobile

fixes CNVS-17096

test plan:
  - call assignment api index for course
    - if you include "all_dates" in the include
      then each assignment has an all_dates field
    - if you dont include it the assignment doesnt
  - call assignment api a single assignment
    - if you include all_dates=true, you get dates
    - if not you dont
  - call assignment_groups api
    - all_dates works like the index api
  - as a teacher and user the right dates show up
    on the assignment index & individual assignment pages

Change-Id: I172c21161f792ff0b272dec03e0eb29ec3f32503
Reviewed-on: https://gerrit.instructure.com/45196
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>