Come up with a more elegant solution for base_url than ds.urls.path()

[simonw]

While fixing #1519 I added a lot of ugly code that looks like this:

datasette/datasette/facets.py

Lines 228 to 230 in 08947fa

	"toggle_url": ds.urls.path(
	path_with_removed_args(self.request, {"_facet": column})
	),

See these two commits in particular: fe687fd and 08947fa

It would be great to come up with a less verbose and error-prone way of handling this problem.