Remove support for named URLs; users can use reverse_lazy instead.

[Osmose]

As mentioned in #164, since users can just use reverse_lazy to turn named URLs into full URLs in the settings file, we don't need the logic for auto-reversing URLs anymore, so I removed it to keep things simple.

[Osmose]

@peterbe r?

[peterbe]

Yes please! The code is perfect. However, I think this lacks some documentation or example or something. I'd love to see this mentioned in the docs.

[Osmose]

Updated with a note in the settings docs about how to use non-URLs for the settings values.

[peterbe]

r+