Using the contact ID (which is about to disappear, anyway) to refer to
contacts in commands makes for an awkward UX.
For a differently awkward UX, use the shortest (>3) unambiguous prefix
of the hostname instead. This is stable, always typable, kind of
memorable, and seems like a decent compromise for now.
ricochet-cli can act as a headless backend, so for now there is no point
to having a separate backend program. It may be reintroduced later for
multiprocess use by other frontends -- to be determined.