app_reverse: Massive simplification, only take the urlconf and the language into account by default
Actually do what the pull request says
app_reverse: Actually fall back to any language if current language does not exist

Thanks for noticing, Marc Egli.
Remove an unnecessary assignment

Thanks Simon Bächler for noticing.
Simplify ApplicationContent.closest_match by moving the caching into app_reverse

Also, only cache (urlconf, prefix) instead of the model.
app_reverse cache: Also use settings.SITE_ID if set
Protect the app_reverse cache against offline database changes
Follow the advice of Simon Bächler and do not use new_*