extract secure-id handling from incoming message processor

fixes CNVS-12147

test plan
- regression test incoming mail

Change-Id: Id3d8a95dda3566bae8d395d1b759c7897ab3c1ee
Reviewed-on: https://gerrit.instructure.com/32671
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>