Fix course copy migration regression
Fix course copy migration regression

The regression prevented users from importing courses using our version
of the plugin. It was introduced in f8400335 when the migrations
controller started checking for valid contexts. This fix is based on
2ba4531f (default_plugins.rb:140).

grahamb/canvas-lms,va7map/canvas-lms,va7map/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,va7map/canvas-lms,grahamb/canvas-lms,va7map/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,sfu/canvas-lms