Fixed #22016 - reload i18n files on change

 * compiles .po files into .mo files if the .mo doesn't exist, or if the .po file is newer than the .mo
 * handled in trans_real, so anything using django.util.translation (like templates) sees the changes
 * also changed the javascript_catalog generation code
 * no server restarts or compilemessages required!
 * unit tests for all of the above