[Request] Add road centerlines to georeferencing interface

[mradamcox]

Describe the feature or enhancement you're looking for
This came up in a Q & A session, and it would really be nice to be able to snap new control points to the intersections of road lines.

Some ideas for how to do this:

1. It could be a WMS (this is already supported if the WMS is from the local GeoNode), and while this wouldn't allow snapping, it would still be nice and easy. Non-roadlines could be added too, like parcel lines from some city GIS server.
2. OSM vectors could potentially be acquired and added to the interface as a layer with snapping. This is super attractive because it would match exactly the OMS street basemap. Performance would be an issue though, so the roadlines would have to be as localized as possible.

Additional context
Certainly this would be a great feature, but it's not a high priority right now.