auth login: add a paste-back mode for machines without a local browser #1364

atgc auth login needs a browser that can reach its local callback on 127.0.0.1. On a remote box with no forwarded port, that fails. The community CLI tng has a --manual mode: it prints the authorization URL and the user pastes back the redirect URL.

Add an equivalent for atgc: print the URL, read the pasted redirect URL from stdin, and finish the exchange.