Deprecate `NavGroup` and move all functionality into `NavItem`

[joshuadavidthomas]

I started out this library in a private repo and went with the simple naive approach of having a NavGroup and NavItem, one being able to have sub-items. But as this library has evolved, it's making less sense to keep them apart.

Recent changes (that have not been published in a new version, yet) have brought them even further together, with NavGroup only differing in a few ways.

[joshuadavidthomas]

This would get rid of the need for the slightly hacky "items": None in NavItem.get_context_data.

[joshuadavidthomas]

I think this is probably the correct way forward, but it will need to be handled gracefully. It's probably too much of a change for the upcoming release (it already has a ton of changes), so we can plan it for the next release.

Let's split it across two releases though:

1. Move all the functionality within NavItem and have a shell NavGroup that logs a warning about the deprecation when used.
2. Remove from the library after a bit of time has passed.

If time permits, a Django check might be nice. But that may be overkill and a warning on the NavGroup would probably suffice.

[joshuadavidthomas]

This approach would not work if we went down the road of using widgets (#102) so one or the other should be chosen