added validations to draft state dialogs

fixes CNVS-11173

test plan:
  - as a teacher with draft state on
  - go to the assignment index
  - create or edit an assignment group via the dialog
    - you should get an error if the name is over 255 chars
    - dialog should otherwise work as expected
  - create or edit an assignment via the dialog
    - you should get an error if the name is over 255 chars
    - dialog should otherwise work as expected

Change-Id: I96f7d1edfb3c182d74110366e1f2d581913864e9
Reviewed-on: https://gerrit.instructure.com/30114
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>