External classifiers

[di]

This PR moves the canonical source for classifiers out of PyPI's database and into the trove-classifiers project.

This is a necessary step to resolve pypa/packaging#147 and move PyPI's metadata validation out of PyPI and into packaging.

PyPI's behavior here for the end user remains unchanged. For administrators, new & deprecated classifiers are no longer created through the UI, but as pull requests for trove-classifiers and dependency upgrades here.

PyPI still retains a Classifier model to help with efficiently searching for projects based on classifiers, but it is now essentially just a mapping from the classifier name to id. New rows in the classifiers table are created for new classifiers the first time a project is uploaded using that classifier.

Fixes #4626, fixes #3786, closes #4950, closes #1244.

[pradyunsg]

Hey @di, are we waiting on reviews from @ewdurbin and possibly others here?

[di]

Need to merge/release pypa/trove-classifiers#6 and update this here first, and waiting on a review from @ewdurbin at least (although we've chatted about it and I don't think there's much else to do here).

[ewdurbin]

Just out of an abundance of caution, I'm gonna snag a backup of the classifiers table before this merges. :-D

Otherwise, 💯

[ewdurbin]

Backup taken!