stop appending digits to long assignment names

fixes CNVS-1662

test plan:
  - turn draft state off
  - go to the asignments index
  - create an assignment group w/ a 254 char name
  - click the add assignment link on the right side
    with the newly created assignment group selected
    - the assignment created should not have a number at the end
    - it should save properly
  - click the add assignment link on the right side
    with a different group (without a long name)
    - the assignment created should have a number at the end
      (like usual)

Change-Id: I1cb7c1aa80eb698e1e5b84098448817e943e79fa
Reviewed-on: https://gerrit.instructure.com/35300
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>