add canvadocs.preferred_plugin_course_id dropped_columns
add canvadocs.preferred_plugin_course_id dropped_columns

Refs: RD-2906
This should have been in e54ff2e0 :(

Change-Id: I17a57e73041a797a4f466e83d6419ac91e9ee3b9
Reviewed-on: https://gerrit.instructure.com/108401
Reviewed-by: Neil Gupta <cc4fec8a34766005dd45d48be045c08d736c11ed@instructure.com>
Tested-by: Jenkins
QA-Review: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Product-Review: Brad Horrocks <7b5c5d1f2eba6c540b8e53f17d87a9bd7dbf604b@instructure.com>

venturehive/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms