Canvas commits

[shaidar]

Below are all the Canvas commits in mitx/juniper that need be combined and submitted as one commit on the mitx/koa branch:

39d51fda48a5bf2895ee1bcf8fe13bd27a8300fd	Sync canvas enrollments
b4a6ea47148f1c821228d1978abfa34fa3ec3a20	Added instructor dashboard button to push edX grades to Canvas
663379d9320a450c59d6ddc3c0ef54c4b1b4f74c	Fixed canvas grade syncing JS
69bfcbca76dcd9cfce16c78140309001d5ce19ed	Changed 'edX' reference to 'MITx'
bb3fc84195dda44f8c1f4bcd7a5685849a5c6e23	Use EDIT_COURSE_ACCESS permission instead of is_staff
58e882077a257671cb2e29cf2ec5f4f0a93a678a	Use OVERRIDE_GRADES rule instead
6bb46a387b91125edb05e2548b8f4710178af75e	Use instructor_tasks for canvas work
1371eefedb8fafc4e343a541baf54578f1cb18a3	Don't return output to prevent task output max size error (#189)

acceptance criteria

• rebase all commits into one
• PR merged into mitx/koa branch

not going to do

• xPRO doesn't integrate with canvas, so there's no point in adding it to xPRO.