fix grading standard can-edit logic

remove worse-than-useless policy block (the controllers are not
using it, checking :manage_grades on the context instead), and
let the view check whether the grading standard has assessed an
assignment, consistent with learning outcomes.

(Also, consistent with learning outcomes, the API can still edit
them. we just make it hard to mess with them accidentally in the
UI. I don't actually like this much, but apparently people
depend on this behavior.)

test plan:
 1. link an assignment to a grading scheme
 2. verify the grading scheme can still be edited
    on the grading schemes page
 3. as a student, submit the assignment
 4. verify the grading scheme can still be edited
 5. as a teacher, grade the student's submission
 6. verify the grading scheme cannot be edited on
    the grading standards page (no pencil icon appears)

fixes CNVS-15933

Change-Id: I21b67a25083169312c8b1c929d8e8ef305adc36c
Reviewed-on: https://gerrit.instructure.com/42234
Product-Review: Hilary Scharton <hilary@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>