* fix type mismatch in third_party_auth migrations
* fix type mismatch in verify_student migrations
* fix type mismatch in video_config migrations
* fix type mismatch in verified_track_content migrations
* fix type mismatch in commercemigrations
* fix type mismatch in xblock_config migrations
* fix type mismatch in course_creators migrations
* fix type mismatch in contentstore migrations
Django 2.0 will make this field required for `ForeignKey` and `OneToOneFields`.
In previous versions the option defaulted to `models.CASCADE` when not
specified. This change should make the deprecation warnings in the current
Django version go away.
The migrations where also modified, but the changes should not cause a change in
the database schema since `models.CASCADE` was already the old default.
