allow external feed urls to be https
allow external feed urls to be https

instead of rolling our own request feed method, just use CanvasHttp,
which already supports https urls

fixes CNVS-17680

test plan:
- go to the course announcements page, add a feed with an https url
- in the console, set it's created_at to before the last post from the
  feed
- run `ExternalFeedAggregator.process`
- it should work, creating a new announcements for the post

Change-Id: I43c48e3520a2129aad91e55ffa7aa8aa82e8814a
Reviewed-on: https://gerrit.instructure.com/46322
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Sean Lewis <e0580b9aacf857d9a3792f1965500f217c0fc9c9@instructure.com>
Tested-by: Jenkins
Product-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>

redconfetti/canvas-lms,samuels410/greatlakes-lms,sfu/canvas-lms,leftplusrightllc/canvas-lms,shidao-fm/canvas-lms,roxolan/canvas-lms,skmezanul/canvas-lms,Rvor/canvas-lms,sfu/canvas-lms,juoni/canvas-lms,tgroshon/canvas-lms,grahamb/canvas-lms,eCNAP/CANVAS,HotChalk/canvas-lms,antoniuslin/canvas-lms,fronteerio/canvas-lms,sdeleon28/canvas-lms,tgroshon/canvas-lms,utkarshx/canvas-lms,matematikk-mooc/canvas-lms,djbender/canvas-lms,efrj/canvas-lms,grahamb/canvas-lms,utkarshx/canvas-lms,Jyaasa/canvas-lms,fronteerio/canvas-lms,wrdsb/canvas-lms,dgynn/canvas-lms,samuels410/greatlakes-lms,ShreniiNepal/shrenii_lmp,kyroskoh/canvas-lms,skmezanul/canvas-lms,HotChalk/canvas-lms,AndranikMarkosyan/lms,kyroskoh/canvas-lms,leftplusrightllc/canvas-lms,shidao-fm/canvas-lms,wrdsb/canvas-lms,narigone/canvas,roxolan/canvas-lms,sfu/canvas-lms,utkarshx/canvas-lms,djbender/canvas-lms,narigone/canvas,sfu/canvas-lms,skmezanul/canvas-lms,antoniuslin/canvas-lms,va7map/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,Jyaasa/canvas-lms,greyhwndz/canvas-lms,Rvor/canvas-lms,redconfetti/canvas-lms,shidao-fm/canvas-lms,va7map/canvas-lms,jay3126/canvas-lms,venturehive/canvas-lms,snehakachroo1/LSI_replica,tgroshon/canvas-lms,fronteerio/canvas-lms,snehakachroo1/LSI_replica,juoni/canvas-lms,HotChalk/canvas-lms,grahamb/canvas-lms,juoni/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,tgroshon/canvas-lms,SwinburneOnline/canvas-lms,SwinburneOnline/canvas-lms,instructure/canvas-lms,AndranikMarkosyan/lms,AndranikMarkosyan/lms,narigone/canvas,matematikk-mooc/canvas-lms,juoni/canvas-lms,antoniuslin/canvas-lms,whalejasmine/canvas-lms,samuels410/greatlakes-lms,sdeleon28/canvas-lms,ShreniiNepal/shrenii_lmp,leftplusrightllc/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,va7map/canvas-lms,SwinburneOnline/canvas-lms,ShreniiNepal/shrenii_lmp,Jyaasa/canvas-lms,eCNAP/CANVAS,SwinburneOnline/canvas-lms,roxolan/canvas-lms,redconfetti/canvas-lms,leftplusrightllc/canvas-lms,instructure/canvas-lms,antoniuslin/canvas-lms,kyroskoh/canvas-lms,redconfetti/canvas-lms,ShreniiNepal/shrenii_lmp,eCNAP/CANVAS,venturehive/canvas-lms,sdeleon28/canvas-lms,dgynn/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,kyroskoh/canvas-lms,skmezanul/canvas-lms,wrdsb/canvas-lms,efrj/canvas-lms,dgynn/canvas-lms,shidao-fm/canvas-lms,greyhwndz/canvas-lms,whalejasmine/canvas-lms,fronteerio/canvas-lms,samuels410/greatlakes-lms,snehakachroo1/LSI_replica,wrdsb/canvas-lms,snehakachroo1/LSI_replica,jay3126/canvas-lms,roxolan/canvas-lms,narigone/canvas,jay3126/canvas-lms,efrj/canvas-lms,whalejasmine/canvas-lms,sdeleon28/canvas-lms,Rvor/canvas-lms,HotChalk/canvas-lms,matematikk-mooc/canvas-lms,grahamb/canvas-lms,greyhwndz/canvas-lms,jay3126/canvas-lms,eCNAP/CANVAS,instructure/canvas-lms,AndranikMarkosyan/lms,greyhwndz/canvas-lms,efrj/canvas-lms,whalejasmine/canvas-lms,matematikk-mooc/canvas-lms,venturehive/canvas-lms,grahamb/canvas-lms,va7map/canvas-lms,Rvor/canvas-lms,dgynn/canvas-lms,Jyaasa/canvas-lms,utkarshx/canvas-lms,instructure/canvas-lms