use last_comment_at in submissions_for_context_codes
use last_comment_at in submissions_for_context_codes

test plan:
* the recent feedback on the dashboard sidebar
 should work as before for submission comments
 from other users

closes #CORE-1216

Change-Id: I23ee73e6a43203ccbd32f8a7139c5a3e53dc1b5c
Reviewed-on: https://gerrit.instructure.com/145149
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Jenkins
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms