move translation files to 'locale' like Repomaker does

The django gettext layout seems to the most standard approach with
Python projects.

diff --git a/po/POTFILES.in b/locale/POTFILES.in
similarity index 100%
rename from po/POTFILES.in
rename to locale/POTFILES.in

diff --git a/po/fdroidserver.pot b/locale/fdroidserver.pot
similarity index 100%
rename from po/fdroidserver.pot
rename to locale/fdroidserver.pot