Fixed #22261 - Added attempt to use reverse() to get the flatpage url.

Falls back to using just the flatpage.url, in case the catchall middleware is used.
Solves 3 use cases for adding flatpages -
* Using the catchall middleware.
* Adding an include(flatware) into your urls.
* Adding flatpages hardcoded (one by one) to your urls.
I don't think strings don't need r"" prefix?