pull canvas specific code out of Facebook class

fixes: CNVS-12520

test plan:
- regression test facebook functionality

Change-Id: I0016edccd3186e7d55146bf216ec2b07a369c801
Reviewed-on: https://gerrit.instructure.com/33531
Reviewed-by: Joel Hough <joel@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>