Follow SHORTER_LANGUAGES when creating app manifests (bug 835407)

This also fixes a bug in the translations app where we were comparing
the lowercase locale to settings.HIDDEN_LANGUAGES which (for
Marketplace) had locales of the form xx-YY. The result was no
translation saved for locales in HIDDEN_LANGUAGES. This updates them to
go through amo.utils.to_language which returns the xx-YY form. It also
switches from using settings.LANGUAGES (lowercase locales) to
settings.AMO_LANGUAGES.