Add JWKS endpoint support to demo.scitokens.org

[jbasney]

The https://demo.scitokens.org/ site is an out-of-date clone of https://jwt.io/ that is customized to implement SciTokens Claims and SciTokens Verification. The source is https://github.com/scitokens/scitokens-heroku which is a fork of https://github.com/jsonwebtoken/jsonwebtoken.github.io.

Currently https://demo.scitokens.org/ and https://jwt.io/ require manually pasting the signing certificate into the form for verification, rather than using the JWKS URI as described in RFC 8414. Adding JWKS URI support to https://github.com/jsonwebtoken/jsonwebtoken.github.io could be a useful enhancement.

https://jwt.davetonge.co.uk/ (https://github.com/davidgtonge/jwt) is an example of a site with JWKS URI support (for comparison).

This is a Javascript project.