gpa scale grading standard

fixes CNVS-8931

makes a new grading standard type called "gpa scale"
only allows for grades on the GPA scale
modifies GB2 dom elements if gpa scaled
modifies rubric_assessment response to properly change grade
adds dialog when selecting gpa scale grading type
refactors grading_standard.rb to use instance methods (not class)
added related tests

test plan:
  - As a Teacher, create an assignment with GPA Scaled grading type
    - Enter a grading scheme like the one in grading_standard.rb/default_gpa_scheme
    - Go to GB2 and grade this assignment
      - grades not in the schema should be "-"
      - letters that aren't in the scheme should be a "-"
      - if "3.0" is in the grading standard an input of 3 should
        switch to 3.0 automatically
      - letters in the scheme should work upper or lower case
      - setting default grade should work with the same rules
      - dom elements of this assignment should have a special class
    - Other assignments should work as usual in GB2
    - Go to Assignment Edit Page
      - when selecting Display Grade as "GPA Scale" a ? icon should pop up
      - clicking this icon should open a modal with sane text
    - Go to GB1
      - Grading an assignment should work like GB2
      - GB1 should work as expected for non GPA Scale Assignments
    - Go to the SpeedGrader
      - grading assignments that are GPA Scaled should work similarly to GB2
      - SpeedGrader should work as expected for non GPA Scale Assignments
    - Enable Rubric based grading on an assignment
      - once the rubric is filled out in SpeedGrader, the correctly rounded GPA should be inserted on save
      - once the rubric is filled out in Submission Edit page, the correctly rounded GPA should be inserted on save
      - rubrics should work as expected for non GPA Scale Assignments
    - Other areas in which GradingStandards are used should work as expected

Change-Id: Ide1ea5ca13a70116ae3535b2c8e105ece79a34c8
Reviewed-on: https://gerrit.instructure.com/29742
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>