localHandle(handle: string,isSelfHost: (host: string) => boolean): string
Returns handle as this instance writes it, isSelfHost telling its own
host. Another instance writes a user from here with this one's host,
bob@gerbeur.example, who is bob here: a mention or a link copied from
there names them so. Any other handle is returned as it is. The server
resolves names by it (resolveUsername, the mention notifications), and
the chat's highlight (mentionsUser) reads mentions by it.