Extract and inject incoming message handler

fixes CNVS-12145

QA Test Plan:
- regression test incoming mail
- ensure that reply to discussion topic works

Change-Id: Iae88aa6da5cfe79e51609e233c05e356feacc198
Reviewed-on: https://gerrit.instructure.com/32610
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>