Update and fix tests

 * Define language in tests
 * Add French to test setup.
 * Refactor tests to be independent on language settings of used
   settings module.
Update multilingual flatpages

 * We also had to rename flatpages application to 'mlflatpages' to avoid
   conflict in application names.
Fix styles in administration
Update documentation
Update function to make language code safe
Refactor tests
Update sanitize language codes in models
Remove logging fallback import
Remove obsolete form factory overrides

 * Remove obsolete factories for multilingual forms.
Refactor core to work with Django 1.6

 * Introduce `TranslationRelation` relation object to handle translation
   logic.
Update flatpages