custom gradebook columns ui

closes CNVS-5856

Test plan:
  * in a course that has n custom gradebook columns, load the gradebook
    * the custom columns should be displayed in the frozen section
    * you should be able to edit the content for those cells and it
      should be remembered
  * enable the teacher notes column (in the gradebook dropdown)
  * use the teacher notes column
  * hide the teacher notes column

Change-Id: Ie7ab048d29047e895f4dac4018bce0d9b3897591
Reviewed-on: https://gerrit.instructure.com/27898
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>