fix order of parts in outgoing multi-part emails
fix order of parts in outgoing multi-part emails

the order of the parts in a multi-part email matters. The last
displayable format is the format email clients will display by
default.

fixes CNVS-9184

test plan:
 - configure canvas to actually send outgoing email.
 - configure a user with a real email address that you can
   access. set notifications for this user to asap.
 - have a different user initiate a conversation with the user
   that has the real email address.
 - check for the notification email from the conversation and
   make sure html format is displayed by default.

Change-Id: I4263bb914ad25d305a0578dc7401d795b2664f80
Reviewed-on: https://gerrit.instructure.com/26215
Reviewed-by: Zach Pendleton <514dbb4d54d4f29e454990df7d075678f43f92aa@instructure.com>
QA-Review: Steven Shepherd <e794458448443812fb2685675aab3bbd1a3891bc@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Product-Review: Jon Willesen <eb5ce5e6a76c243326fecfa94534101600f87bd9@instructure.com>

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