* Fix type mismatches in track migrations
* Fix type mismatches in oauth_dispatch
* Fix type mismatches in badges migrations
* fix type mismatch in contentserver migrations
* Fix type mismatches in mobile_api migrations
* fix type mismatch in crawlers migrations
* fix type mismatch in dark_lang migrations
* fix type mismatch in branding 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.
