uses conversation context tags for notifications

fixes CNVS-11269

test plan
- start conversations and send messages from new and old
 conversations between users who share no contexts, one context,
 two context, and >2 contexts
- ensure that the notification emails include shared courses next
 to the author's name
- ensure that messages sent from new conversations only include
 the selected context next to the author name

Change-Id: Ibc05752100dfb4b139c8dce64cfe1985987c6467
Reviewed-on: https://gerrit.instructure.com/30778
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>