Facilitate navigation

[meshy]

Currently, the data model doesn't allow for routes within a level of a hierarchy to be organised. They will always be output in alphabetical order. We could switch over to using MPTT on the Route model for this, but I'd rather not. Perhaps I'm just being stubborn.

Separating the navigation from the Route model would allow us to represent nav structures that differ from the URL hierarchy. For example, "Home" (/) may be presented alongside "About" (/about/), and "News" (/news/) despite being on a different level.

MPTT may be a good fit for this (on a separate model, perhaps Navigation), but if there are alternative (ideally simpler) algorithms, I'd love to know about them.