Create an explicit list of all Release Manager GPG keys

[sethmlarson]

Instead of recommending users import GPG keys by ID (a process which can be fallible) we should provide a file containing all public keys of release managers both current and historical and document a direct import method to GPG instead of by key ID.

This will also be useful for future folks who may find it difficult to discover the authoritative keys.

[JacobCoffee]

This would be done in the backend at https://www.python.org/admin/boxes/box/57/change/