Frequent locales problems for new users

[Leo Famulari]

Warning! Locales! New users seem to have trouble with Guix locales every
day.

I think we can improve the situation.

First, we can deprecate the glibc-utf8-locales package and not mention
it in the manual section Application Setup. I've seen users think they
had to install it in order to get UTF-8 support. Everyone should be
using glibc-locales. Eventually we can rename it to
'glibc-locales-for-tests', and hide the package too.

Second, we need to make sure that guix-install.sh is setting up
GUIX_LOCPATH correctly. I see that the binary tarball's store includes
glibc-utf8-locales, so it should be possible for things to "just work",
ignoring that it's the wrong locales package. Does anyone know any
particular issues with the installer that would cause trouble?