srgb - persist assignment sorting

fixes CNVS-12101
fixes CNVS-12326

test plan
- in gb2, set sort order to due date
- go to srgb, verify that assignment sort order is set to due date
- verify that assignments are sorted in the correct order (due date)
- in srgb, change setting to alphabetical
- verify that srgb sorts assignments alphabetically
- go to gb2, verify that it doesn't blow up
- verify that assignments are sorted by assignment group (*not* alpha)
- go back to srgb, verify that assignments are sorted alphabetically still

notes
- gb2 should be able to sort and save sort preference by due date, assignment group, or custom
- srgb should be able to sort and save sort preference by due date, assignment group, alphabetically
- due date and assignment group sorting should persist across both gradebooks
- both gradebooks should default to assignment group sorting if presented with a sort setting that it cannot process

Change-Id: I5b2fcc60ecdd9a1f3932f1d25d0459d5421cb3a1
Reviewed-on: https://gerrit.instructure.com/32954
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>