change migration to be postdeploy

refs g/42395

test plan
 - migrations should run

Change-Id: Icb302d1b26c4cabed1bbb3dd25dad31b7d381936
Reviewed-on: https://gerrit.instructure.com/44106
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>