fix messageable_users query for mysql

fixes CNVS-5067

test-plan:
  - use mysql database
  - create an account admin
  - create a course with a teacher in that admin's account
  - as the account admin
    - go to the conversation inbox
    - open the recipient browser for a new conversation
    - search for the teacher
  - should find the teacher without any server errors

Change-Id: Iff383bb8eff62dd1caf27f57b3300675a1ca1dbb