added max length to srbg and GB2 notes

fixes CNVS-11062

test plan:
  - go to srgb
    - add a long note to custom columns, it should stop at 255 characters
    - when you reload the page the note should be persisted at the same length
  - go to gb2
    - add a long note to custom columns, it should stop at 255 characters
    - when you reload the page the note should be persisted at the same length
  - notes should otherwise work as usual

Change-Id: I9cd1b86e87d08100c25d9543f5eceeed685c5afe
Reviewed-on: https://gerrit.instructure.com/32623
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>