Add `Route.get_parent`

[meshy]

We have get_ancestors, but we can't so easily get the immediate one.

[meshy]

Question 1

We have only one route:

route = Route.objects.create(url='/anything/')

If we call route.get_parent(), do we get None? Would it be more sane to raise DoesNotExist?

Question 2

We have only two routes:

r1 = Route.objects.create(url='/')
r2 = Route.objects.create(url='/nested/twice/')

If we call r2.get_parent(), do we get r1?