Fix python3 migration issue #1287

[alles-klar]

Related to #1287

During the python3 migration 'product_grade_b' was accidentally renamed to 'product_grade_'. This PR reverts the renaming.

This PR may break current installation which are based on the dev branch (python3) but makes the current branch compatible with installations which are based on the master branch.

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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
• 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.

[devGregA]

@alles-klar This is great catch, but I'm worried it may have some issues with installs currently running. Let me work through the use case then I'll merge.