Update migrations to make builds happy

[Maffooch]

Note: DefectDojo is now on Python3 and Django 2.2.1 Please submit your pull requests to the 'dev' branch as the 'legacy-python2.7' branch is only for bug fixes. Any new features submitted to the legacy branch will be ignored and closed.

When submitting a pull request, please make sure you have completed the following checklist:

• Your code is flake8 compliant
• Your code is python 3.5 compliant
• If this is a new feature and not a bug fix, you've included the proper documentation in the ReadTheDocs documentation folder. https://github.com/DefectDojo/Documentation/tree/master/docs or provide feature documentation in the PR.
• Model changes must include the necessary migrations in the dojo/dd_migrations folder.
• Add applicable tests to the unit tests.