speed up user note permission checks on the inbox

fixes CNVS-14990

test plan
- have a user associated with many accounts (~10)
- have teacher enrollments in many courses in those accounts (~100)
- conclude a significant portion of the enrollments (~75%)
- enable user notes on at least one of the accounts
- make sure the user does not have :manage_students permissions on
 any of the accounts
- load the conversation inbox
- ensure that the inbox loads in a timely fashion (at least faster
 than without this commit)
- regression test user notes in conversations

Change-Id: I9d108f193c998e970c3798181dc3faf9a13cadf4
Reviewed-on: https://gerrit.instructure.com/39756
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>