client: fix for Puavo REST API breakage! Again!

Pulseaudio failed to start because group names in
/var/lib/extrausers/group were broken (fchown for runtime dir failed:
Invalid argument). Puavo REST API has been changed to convey
POSIX-groups in abbreviation-field instead of name-field.
client: set the default encoding to UTF-8 before reading from file
client: disable cupsd browsing on laptops

Printers are added from puavo_session.json
client: create hosttype and version specific logo for lightdm and webmenu