Make is_anonymous_author read only on discussion
Make is_anonymous_author read only on discussion

refs VICE-2348
flag=partial_anonymity

Test Plan:
 - in a rails console try to modify is_anonymous_author
 > should not be able to

Change-Id: I9691ca1f70e65152f8a8a1cd747c8615dc69a170
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/281745
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Chawn Neal <908eabd8c84ff6c7c139535b8d37802d58ad1683@instructure.com>
QA-Review: Chawn Neal <908eabd8c84ff6c7c139535b8d37802d58ad1683@instructure.com>
Product-Review: Chawn Neal <908eabd8c84ff6c7c139535b8d37802d58ad1683@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms