Implement support for multiple aerial imagery layers

I'm sure this is something that's planned eventually, but since I don't see an actual issue about it; I figured I'll create one.

It would be nice if we could have a button to switch to Aerial imagery of some kind. I don't know if using the editor layer index would be allowed but it would be convenient of possible since it already contains so many layers. https://github.com/osmlab/editor-layer-index

But as a first step, being able to manually add aerial imagery tile urls would be nice