don't show empty conversations in new inbox
don't show empty conversations in new inbox

fixes CNVS-8116

test plan
- you will need a student and a teacher who do not have a
 conversation with each other
- create an assignment
- as a student, make a submission
- make a submission comment on that submission
- as a teacher, reply to that submission comment
- ensure that a conversation with the submission comments and no
 other messages appears in the teacher's old conversations inbox
- ensure that the conversation does not appear in new conversations

Change-Id: I046c387d39b2e8affcb6572cdb5e049d666bdc9f
Reviewed-on: https://gerrit.instructure.com/24486
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Zach Pendleton <514dbb4d54d4f29e454990df7d075678f43f92aa@instructure.com>
QA-Review: Cam Theriault <b4133c36b6d893d0872cc09e87b536d16aa2d241@instructure.com>
Product-Review: Joel Hough <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@instructure.com>

grahamb/canvas-lms,Jyaasa/canvas-lms,greyhwndz/canvas-lms,sfu/canvas-lms,jay3126/canvas-lms,wrdsb/canvas-lms,sfu/canvas-lms,ShreniiNepal/shrenii_lmp,narigone/canvas,arrivu/hoodemo,efrj/canvas-lms,kyroskoh/canvas-lms,instructure/canvas-lms,landonwilkins/canvas-lms,utkarshx/canvas-lms,dgynn/canvas-lms,kyroskoh/canvas-lms,Rvor/canvas-lms,arrivu/hoodemo,jay3126/canvas-lms,juoni/canvas-lms,wrdsb/canvas-lms,arrivu/qibla-ecampus,samuels410/greatlakes-lms,leftplusrightllc/canvas-lms,sdeleon28/canvas-lms,sfu/canvas-lms,AndranikMarkosyan/lms,sdeleon28/canvas-lms,landonwilkins/canvas-lms,efrj/canvas-lms,antoniuslin/canvas-lms,SwinburneOnline/canvas-lms,landonwilkins/canvas-lms,greyhwndz/canvas-lms,loganrun/pincan,samuels410/greatlakes-lms,wimemx/Canvas,roxolan/canvas-lms,grahamb/canvas-lms,wye0220/shindig-canvas,arrivu/arrivu-lms,Unow/canvas-lms,wye0220/shindig-canvas,instructure/canvas-lms,skmezanul/canvas-lms,fronteerio/canvas-lms,usmschuck/canvas,dgynn/canvas-lms,sfu/canvas-lms,leftplusrightllc/canvas-lms,tgroshon/canvas-lms,arrivu/qibla-ecampus,wye0220/shindig-canvas,redconfetti/canvas-lms,jay3126/canvas-lms,Rvor/canvas-lms,HotChalk/canvas-lms,Rvor/canvas-lms,usmschuck/canvas,sfu/canvas-lms,Unow/canvas-lms,snehakachroo1/LSI_replica,sdeleon28/canvas-lms,va7map/canvas-lms,redconfetti/canvas-lms,SwinburneOnline/canvas-lms,usmschuck/canvas,instructure/canvas-lms,sdeleon28/canvas-lms,ShreniiNepal/shrenii_lmp,leftplusrightllc/canvas-lms,snehakachroo1/LSI_replica,utkarshx/canvas-lms,HotChalk/canvas-lms,roxolan/canvas-lms,loganrun/pincan,fronteerio/canvas-lms,shidao-fm/canvas-lms,juoni/canvas-lms,djbender/canvas-lms,redconfetti/canvas-lms,venturehive/canvas-lms,landonwilkins/canvas-lms,redconfetti/canvas-lms,grahamb/canvas-lms,utkarshx/canvas-lms,matematikk-mooc/canvas-lms,eCNAP/CANVAS,usmschuck/canvas,venturehive/canvas-lms,greyhwndz/canvas-lms,ShreniiNepal/shrenii_lmp,tgroshon/canvas-lms,AndranikMarkosyan/lms,HotChalk/canvas-lms,va7map/canvas-lms,shidao-fm/canvas-lms,matematikk-mooc/canvas-lms,va7map/canvas-lms,antoniuslin/canvas-lms,whalejasmine/canvas-lms,kyroskoh/canvas-lms,skmezanul/canvas-lms,Jyaasa/canvas-lms,Jyaasa/canvas-lms,tgroshon/canvas-lms,roxolan/canvas-lms,HotChalk/canvas-lms,eCNAP/CANVAS,sfu/canvas-lms,samuels410/greatlakes-lms,dgynn/canvas-lms,matematikk-mooc/canvas-lms,Jyaasa/canvas-lms,grahamb/canvas-lms,leftplusrightllc/canvas-lms,gbleydon/canvas-lms,venturehive/canvas-lms,eCNAP/CANVAS,kyroskoh/canvas-lms,gbleydon/canvas-lms,juoni/canvas-lms,greyhwndz/canvas-lms,SwinburneOnline/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,wimemx/Canvas,grahamb/canvas-lms,AndranikMarkosyan/lms,snehakachroo1/LSI_replica,whalejasmine/canvas-lms,ShreniiNepal/shrenii_lmp,antoniuslin/canvas-lms,eCNAP/CANVAS,shidao-fm/canvas-lms,matematikk-mooc/canvas-lms,utkarshx/canvas-lms,tgroshon/canvas-lms,gbleydon/canvas-lms,djbender/canvas-lms,fronteerio/canvas-lms,wrdsb/canvas-lms,arrivu/hoodemo,venturehive/canvas-lms,SwinburneOnline/canvas-lms,roxolan/canvas-lms,AndranikMarkosyan/lms,instructure/canvas-lms,narigone/canvas,samuels410/greatlakes-lms,loganrun/pincan,juoni/canvas-lms,antoniuslin/canvas-lms,narigone/canvas,djbender/canvas-lms,wimemx/Canvas,efrj/canvas-lms,efrj/canvas-lms,sfu/canvas-lms,dgynn/canvas-lms,arrivu/arrivu-lms,whalejasmine/canvas-lms,jay3126/canvas-lms,va7map/canvas-lms,whalejasmine/canvas-lms,Rvor/canvas-lms,skmezanul/canvas-lms,fronteerio/canvas-lms,narigone/canvas,instructure/canvas-lms,wrdsb/canvas-lms,djbender/canvas-lms,Unow/canvas-lms,skmezanul/canvas-lms,snehakachroo1/LSI_replica,arrivu/qibla-ecampus,arrivu/arrivu-lms,shidao-fm/canvas-lms