gradebook2 assignment_group weight changes without reload

fixes CNVS-10135

Note: changing weight will break drop rules, this
is being taken care of in another ticket.

Test Plan:
  - make several assignment groups w/ weights
  - go to GB2
  - change weights in modal window
    - ensure weight %s change in headers
    - ensure grades change appropriately
  - toggle weighted groups on/off
    - ensure percentages appear/disappear
    - ensure grades change properly
  - make an assignment group named "<h1>Foo</h1>"
    - play around with weights in gradebook
    - ensure proper escaping in headers and dialogs

Change-Id: I771e27a1a07958dba3fa9cae2c8da11aed3371a7
Reviewed-on: https://gerrit.instructure.com/28075
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Mike Nomitch <mnomitch@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>