[join] integrated join through testbed workflow

[yoursunny]

There needs to be an integrated "join through testbed" workflow, to reduce friction.

1. User clicks on a link like https://ndn-workspace.web.app/join-via-testbed?anchor=/8=ndn/8=weekly-call-doc/8=KEY/8=%A0w%E9%88%23%27%D91/8=self/54=%00%00%01%92%CA%C4EN/1=%16u%A9%29%8BM%DA%82%A5%8FR%F2%AB%04-%89a%CF%82%AF%ED%FD%A1%5B%0E%28%F0%23%27%0B%D5%24
   The weekly email should refer to this link.
2. The application automatically guides the user to obtain a testbed certificate, if one does not already exist.
3. The application automatically starts the proof-of-possession process, if there isn't an existing profile in the same workspace.