re-check browser support if user_agent changes, fixes CNVS-5025
re-check browser support if user_agent changes, fixes CNVS-5025

previously we stored whether your browser is supported in the session
(so as not to add extra overhead to every request). we still do that, but
now we re-check whenever the user_agent string changes

test plan:
1. install an older chrome or firefox
2. log in to canvas
3. see the orange bar
4. upgrade your browser
5. you should not see the orange bar in canvas
6. additionally, if you change your user_agent string on the fly, the
   orange bar should show or not show accordingly when you refresh

Change-Id: I5c21c3c7adf30d4dd5b90189c3d9b3a33f8fa33f
Reviewed-on: https://gerrit.instructure.com/19189
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Stanley Stuart <2b43fb8b7a234825d50dd49ce7892d78a59da8f3@instructure.com>
QA-Review: Cam Theriault <b4133c36b6d893d0872cc09e87b536d16aa2d241@instructure.com>
Product-Review: Bryan Madsen <0a3ba1af216659500dd8f41e29f3979f42452870@instructure.com>

roxolan/canvas-lms,leftplusrightllc/canvas-lms,venturehive/canvas-lms,redconfetti/canvas-lms,Rvor/canvas-lms,sdeleon28/canvas-lms,antoniuslin/canvas-lms,Unow/ins,leftplusrightllc/canvas-lms,va7map/canvas-lms,SwinburneOnline/canvas-lms,arrivu/hoodemo,whalejasmine/canvas-lms,wimemx/Canvas,efrj/canvas-lms,leftplusrightllc/canvas-lms,Jyaasa/canvas-lms,va7map/canvas-lms,leftplusrightllc/canvas-lms,HotChalk/canvas-lms,venturehive/canvas-lms,antoniuslin/canvas-lms,usmschuck/canvas,samuels410/greatlakes-lms,tgroshon/canvas-lms,ShreniiNepal/shrenii_lmp,grahamb/canvas-lms,tgroshon/canvas-lms,sdeleon28/canvas-lms,juoni/canvas-lms,jay3126/canvas-lms,djbender/canvas-lms,HotChalk/canvas-lms,utkarshx/canvas-lms,redconfetti/canvas-lms,arrivu/hoodemo,efrj/canvas-lms,jay3126/canvas-lms,greyhwndz/canvas-lms,wrdsb/canvas-lms,shidao-fm/canvas-lms,djbender/canvas-lms,Unow/canvas-lms,arrivu/qibla-ecampus,grahamb/canvas-lms,redconfetti/canvas-lms,sli7246/canvas_branch,sfu/canvas-lms,juoni/canvas-lms,va7map/canvas-lms,samuels410/greatlakes-lms,ShreniiNepal/shrenii_lmp,utkarshx/canvas-lms,arrivu/hoodemo,sfu/canvas-lms,samuels410/greatlakes-lms,skmezanul/canvas-lms,landonwilkins/canvas-lms,wye0220/shindig-canvas,arrivu/jigsaw-lms,tgroshon/canvas-lms,wimemx/Canvas,greyhwndz/canvas-lms,whalejasmine/canvas-lms,shidao-fm/canvas-lms,sfu/canvas-lms,loganrun/pincan,arrivu/jigsaw-lms,usmschuck/canvas,kyroskoh/canvas-lms,redconfetti/canvas-lms,roxolan/canvas-lms,Jyaasa/canvas-lms,skmezanul/canvas-lms,kyroskoh/canvas-lms,wye0220/shindig-canvas,fronteerio/canvas-lms,loganrun/pincan,kyroskoh/canvas-lms,roxolan/canvas-lms,landonwilkins/canvas-lms,usmschuck/canvas,merichar/canvas-lms,wimemx/Canvas,fronteerio/canvas-lms,venturehive/canvas-lms,Jyaasa/canvas-lms,arrivu/arrivu-lms,usmschuck/canvas,va7map/canvas-lms,landonwilkins/canvas-lms,antoniuslin/canvas-lms,greyhwndz/canvas-lms,wrdsb/canvas-lms,Unow/canvas-lms,wye0220/shindig-canvas,jay3126/canvas-lms,skmezanul/canvas-lms,Unow/canvas-lms,Rvor/canvas-lms,ShreniiNepal/shrenii_lmp,sfu/canvas-lms,utkarshx/canvas-lms,arrivu/qibla-ecampus,sdeleon28/canvas-lms,greyhwndz/canvas-lms,fronteerio/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,shidao-fm/canvas-lms,SwinburneOnline/canvas-lms,merichar/canvas-lms,sli7246/canvas_branch,fronteerio/canvas-lms,whalejasmine/canvas-lms,arrivu/jigsaw-lms,sfu/canvas-lms,samuels410/greatlakes-lms,juoni/canvas-lms,HotChalk/canvas-lms,Jyaasa/canvas-lms,djbender/canvas-lms,Rvor/canvas-lms,sdeleon28/canvas-lms,landonwilkins/canvas-lms,efrj/canvas-lms,skmezanul/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,sli7246/canvas_branch,jay3126/canvas-lms,Rvor/canvas-lms,merichar/canvas-lms,antoniuslin/canvas-lms,SwinburneOnline/canvas-lms,grahamb/canvas-lms,arrivu/arrivu-lms,roxolan/canvas-lms,efrj/canvas-lms,shidao-fm/canvas-lms,ShreniiNepal/shrenii_lmp,kyroskoh/canvas-lms,whalejasmine/canvas-lms,loganrun/pincan,Unow/ins,wrdsb/canvas-lms,tgroshon/canvas-lms,djbender/canvas-lms,arrivu/qibla-ecampus,utkarshx/canvas-lms,Unow/ins,wrdsb/canvas-lms,arrivu/arrivu-lms,venturehive/canvas-lms,HotChalk/canvas-lms,juoni/canvas-lms