Only download files we have translated resources for (#508)
Only download files we have translated resources for (#508)

Previously, if project had 4 .properties files and 1 .lang file, we
tried to download all files for all locales, even if the .lang file
was only enabled for a handful of them. Since the .lang file didn't
exist, download failed.
mozilla/pontoon,mastizada/pontoon,mozilla/pontoon,mathjazz/pontoon,participedia/pontoon,participedia/pontoon,mastizada/pontoon,mastizada/pontoon,mathjazz/pontoon,mathjazz/pontoon,jotes/pontoon,participedia/pontoon,participedia/pontoon,jotes/pontoon,mozilla/pontoon,mozilla/pontoon,jotes/pontoon,mozilla/pontoon,mathjazz/pontoon,jotes/pontoon,mastizada/pontoon,mathjazz/pontoon