Instant share between two logged-in Talkilla users.

Implementation of the https://trello.com/c/lPfhY1Lc/ card.  Somewhat reframed and simplified from the original card.  Not hidden behind an "alpha-test" mode, as we decided that putting together an alpha-test mode will get much easier soon once there's a gear menu for settings, and that it wasn't too important here.

This probably wants reviewing by either @n1k0 or @Standard8.