UI for adding SIS token to canvas

fixes SIS-641

Test plan:
- generate a token in SIS App
- copy and paste the token in the 'SIS Token Authentication' field
-- you must enable the Post Grades to SIS feature to see this setting
- update the settings and confirm the sis token was saved

Change-Id: I13f2e0136809c42d7b7c7b30c330efb11f6dc29f
Reviewed-on: https://gerrit.instructure.com/44358
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Tyler Pickett <tpickett+gerrit@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Nick Houle <nhoule@instructure.com>