custom columns in srgb

fixes CNVS-10023

test plan
=========
- in a course that hasn't had the notes column created yet, do the
  following:
  - navigate to srgb
  - select a student
  - check the "Show Notes in Student Info"
  - verify that the "Notes" textarea displays
  - reload or navigate to gb2 and verify that this option stays after a
    refresh
  - enter some text in the text area and then navigate away from it
  - refresh to verify your changes were saved to the server
- repeat the above in a course that had the notes column already created
  in the gradebook (and possibly other columns via the api)
- in a class with custom gradebook columns, verify the above are also
  true for each of those
- select a student quickly after page load and verify that textareas are
  disabled while loading column info
- verify that columns with content update when that content loads and
  the textarea is no longer disabled

- in a course with at least one additional custom column, go to gb2,
  show the notes column and reorder the custom columns so that notes
  isn't first
- load srgb, and verify that the custom columns are in the proper order
- after hiding and showing the notes column, verify that it is shown
  first in the custom columns section

Change-Id: I21a81e1c27ed4fd1ae8224529f808cb786ca652f
Reviewed-on: https://gerrit.instructure.com/29048
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Matthew Irish <mirish@instructure.com>