add user note flag to conversations create

refs PS-1497

test plan:
  1. valdate api documentation is updated with user_note
     flag
  2. create a new conversation with a single user adding
     the user_note flag and make sure a faculty journal
     entry is created for that user when the conversation
     is created
  3. create a new conversation with multiple student users
     with the user_note flag and validate that faculty journal
     entries were created for those student users
  4. create a new conversation with non student users and
     the user_note flag and validate faculty journal entries
     were not created for those users

Change-Id: I177402c24bee15c8bc9cc56538d6769b8730b4ad
Reviewed-on: https://gerrit.instructure.com/35218
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ethan Vizitei <evizitei@instructure.com>
Product-Review: Adam Phillipps <adam@instructure.com>
QA-Review: Adam Phillipps <adam@instructure.com>