mailman: leave used_languages alone, use site_languages

used_languages is not meant for end user setting and might get changed after a
"dpkg-reconfigure mailman" which in turn is what runs when puppet notices its
value has changed, which means it'd get executed at every puppet run.

  $ echo set mailman/used_languages "ar big5 ca cs da de en es et eu
  fa fi fr gb hi hr hu ia id it ja ko lt nl no pl pt pt_BR ro ru sl sr sv ta tr
  uk vi zh_CN zh_TW" | debconf-communicate
  0 value set
  $ dpkg-reconfigure -fnoninteractive mailman
  [etc etc]
  $ echo get mailman/used_languages | debconf-communicate
  0 ar ast ca cs da de en es et eu fi fr gl he hr hu ia it ja ko lt nl no pl pt
  pt_BR ro ru sk sl sr sv tr uk vi zh_CN zh_TW
  [different value!]

This change sets site_languages which is what the user gets asked upon mailman
installation and is not supposed to change.

Also remove missing/renamed languages "gb" and "big5".

Change-Id: I0850fe52677e74edcde23cf7985135c80ee1db34
Merge "mailman: leave used_languages alone, use site_languages" into production