Add "replaced by" field for deprecated classifiers

[di]

One problem with our current ability to deprecate trove classifiers is that we don't have a great way to indicate to the user which classifier(s) they should be using instead.

For example, when I deprecated the classifier Natural Language :: Ukranian due to a typo and added Natural Language :: Ukrainian, any user that might have had the old classifier would have received an error like:

HTTPError: 400 Client Error: Invalid value for classifiers. Error: Classifier
'Natural Language :: Ukranian' has been deprecated,
see https://pypi.org/classifiers/ for a list of valid classifiers. for url:
http://upload.pypi.org/legacy/

It would be great if we could optionally identify one or more classifiers which replace a deprecated classifier when deprecating it, so we could make this error message something like:

HTTPError: 400 Client Error: Invalid value for classifiers. Error: Classifier
'Natural Language :: Ukranian' has been deprecated, and replaced with
the following classifier(s): 'Natural Language :: Ukrainian'. for url:
http://upload.pypi.org/legacy/

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[Man-Jain]

@di I would like to work on issue.

[brainwane]

@Man-Jain If you're still interested in this issue, yes, please go ahead!

@nlhkabu heads-up in case we should expose this in some way in the UI in the future.

[xni]

@di, I'd like to work on this issue from the Bloomberg London PyPA Sprint

[xni]

This is the first part, where I'm adding deprecation alternative to the admin page: #4950

I will try to complete the second part (message for the API) tomorrow.