core, frontends: fix invalid translatable strings. Some f-strings where used in translatable text, this has been fixed by using explicit `format()` call (using a script based on `tokenize`). As tokenize messes with spaces, a reformating tool (`black`) has been applied to some files afterwards.

FROM libervia/prosody:e2e

LABEL maintainer="Goffi <tmp_dockerfiles@goffi.org>"

RUN prosodyctl register demo server1.test demo