validate uniqueness of sis_source_id on course
validate uniqueness of sis_source_id on course

Change-Id: Ib4ea0ba921abe5f7708b9a0eb473c8a2afc73d2c
Reviewed-on: https://gerrit.instructure.com/150501
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Product-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
QA-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>

sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms