[web] Simplify a bit language handling
Drop `set_default_language` and `set_user_language` by considering that:
* before being bound to the user, request will have the site's default language
* cnx has the user's preferred language, hence get it back to the request when
it's bound to a connection
The first point requires some change to cubicweb's vreg faking so it doesn't
break.
That should be enough.
etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances