Make UserEmail entirely separate from repository related data

While trying to drop some invalid (users.alioth.debian.org) email from the database, I managed to remove some SourcePackage, SourcePackageRepositoryEntry, DebianContributor, and a few other... this was really undesired and is the consequence of the fact that we have ContributorName linking to UserEmail.

We should refactor the code base so that UserEmail has no link with any data coming from package repositories in general. This is most likely also needed from a GDPR point of view. We want to make sure we can remove user data without breaking our copy of data coming from the repositories.