add subject to conversation html email notification

fixes CNVS-14220

test plan
- cause a conversation message notification to be sent
- check the html email notification that was sent
- ensure that the conversation subject appears in the message

Change-Id: Ie2e1c7cb2865e2cb9a1ac1763ade2eebd88dc373
Reviewed-on: https://gerrit.instructure.com/37716
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>