round group_weights to two decimals

fixes CNVS-10309

Test Plan:
  - Go to old assignments index as teacher
  - Change group weights decimals to have many digits
  - Ensure:
    - Group weight gets rounded to two digits
    - Total weight is rounded to two digits

Change-Id: Idcedb1ea77a0d5bb4e3a14ea979d95c0197d52ba
Reviewed-on: https://gerrit.instructure.com/28325
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>