Add a rake task to reset all email and private tokens

I think this is more of a backstage change. We've already run both tasks on production. Related: https://gitlab.com/gitlab-com/infrastructure/issues/1412

In the future we may have to run this again so makes sense to add them to the code...

Not sure about what milestone should be put this on, though 🤔

I've removed the @gitlab.com user tasks, since I thought this would have no benefit for end users....