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