gradebook url changes

fixes CNVS-11966

test plan:
  - with srgb feature flag enabled
    - cant get to gb1
    - /gradebook takes you to your preference
    - switching saves the preference
    - student behavior is normal

  - with srgb feature flag disabled
    - cant get to srgb
    - /gradebook goes to gb1
    - /gradebook2 goes to gb2
    - student behavior is normal

  - make sure all links to the GBs work
  - student grade summary should work as usual
  - with both FF settings, as a teacher checking a
    single student's grade should work as usual
  - courses with 'large rosters' get GB2 not GB1

Change-Id: I2a488fdfb5b3667c1d6cd21dc250ef3c50df101c
Reviewed-on: https://gerrit.instructure.com/33178
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>