fix migrations for mysql

fixes CNVS-13306
fixes gh-453

MySQL refuses to change columns that have a foreign key on them

Test plan
- Run the polling migrations on MySQL
- They should migrate the database correctly.

Change-Id: Ib781c5237c0973b254043096e536ac094dd809cf
Reviewed-on: https://gerrit.instructure.com/35511
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>