curving gpa_scale grades works

fixes: CNVS-12707

also srgb doesnt show curve grades button
if there are no possible points

test plan:
  - make an assignemnt that is gpa_scale
  - populate gb2 with grades
  - curve grades
    - all the grades shouldn't disappear
    - grades should make sense
  - non gpa_scale curving should work
  - curving on srgb should work as well

  - if an assignment has no points possible
    - neither gb shows the curve button/link

Change-Id: I59bc736da1095ee508bf7fa17d5097fa05eacab6
Reviewed-on: https://gerrit.instructure.com/34002
Reviewed-by: Tyler Pickett <tpickett+gerrit@instructure.com>
QA-Review: Tyler Pickett <tpickett+gerrit@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>