[code cleanup] Add Team slug field removal migration

[qbey]

Code Cleanup

Is your feature request related to a problem or unsupported use case? Please describe.
#480 removes slug field for Team, we must add the migration after the deployment.

Describe the solution you'd like
The best solution should be to use https://github.com/charettes/django-syzygy but for now, the migration was "just" not added. We should add it when the current code has been deployed.

Describe alternatives you've considered
See https://github.com/charettes/django-syzygy quite straightforward, but needs to update the helm charts.

Discovery, Documentation, Adoption, Migration Strategy

Do you want to work on it through a Pull Request?
Will do