implement support for a token/url based approach, to make this much more robust

[williamstein]

For example, the cocalc-byoc client would start by visiting a URL such as

 https://cocalc-server/byoc/nrWGwisgtW4XACmM?ssh=ssh-ed25519%20%AAAAC3NzaC1lZDI1NTE5AAAAIO4a25Oi/jeMe9bWccKc7AMS8lXVf2tjX2xJcgliI1cC%20%cocalc-byoc@blah

and there it would:

• report its public ssh key for this session, and

it would find out:

• the ssh address is (hostname and port, which could both be configured)
• the project id

This would make it so no copy/paste is necessary, which would of course be vastly better. Also, it makes the input to start the cocalc-byoc client much simpler. You just pass it

    https://cocalc-server/byoc/nrWGwisgtW4XACmM

Also, we could make the URL https://cocalc-server/byoc/nrWGwisgtW4XACmM with no query param be something with some user-friendly instructions, in case somebody gets this url and just pastes it into their browser.