Add missing migration

[vasilty]

The migration was missed in https://github.com/Aleksi44/wagtailsvg/commit/9b51a753e687bddc797f3ca0dfb7ae881bb2ce47.

Just a suggestion, an automatic check for missing migrations based on ./manage.py makemigrations --check can be configured, for example with Github actions.

[Aleksi44]

Thanks @vasilty