Adds a user toggle in the preferences menu:
> Show full username (including domain) for remote users
It only shortens the username of local accounts. The main reason for this is so that there is a clear visual indicator of who is local on a thread -- this is important for local-only posting reasons. But we've been using it on Friend Camp for 3 years now and it's actually really nice for getting a sense of who is on what server, too.
The bulk of this work was done by Callie on Friend Camp in October 2019.
Fixes#1247