Create a submodule to handle per-user RSA keys

Just dumping in here what I had started on - not sure how much is working.

This could also be a separate project, but not sure which makes more sense?

ed25519 is an emerging elliptic curve standard that could be a good choice to support, though it's not in the standard.

However, only the NIST curves are supported in the official JWT standard:
https://tools.ietf.org/html/rfc7518#section-3

This other JWT library supports it, but let's try just RSA for now and possibly make a PR on the firebase library to add support.
https://github.com/web-token/jwt-signature-algorithm-eddsa

Here's some more progress, switch to trying to use the user data service for storing keys.

More progress. Starting on Kernel test.

using new fallback header name per #3032702: Fallback header for use when basic authorization header is also sent

With more added test cases in Kernel test

WIP for building forms

better forms

Add form to generate RSA key

ready for initial review.

oops, needed to use the full URL becuase testbot is in a subdirectory. Also added more test assertions.

Annoying, passes locally. try without a check for redirect.

Ok, IDK why the testbot doesn't redirect, but it's not that important.