i18n package resource resolving depends too much on available locale

Instead of just depending on the session's locale, our i18n resolving algorithm should look at the form of the file names as well. For instance, if we get a request for /web/resources/foo.Bar/test_en_US.css, it should recognize that test is the base form, and try en and en_US on top of that, regardless of the locale that is was set in the session.