Easy aerial imagery loader

[okdistribute]

Should be easy to load settings and maps.

Currently, we have a way to import "configuration files" and "offline map tiles" from the File menu.

However, this does not create a style.json file for the Observations view, which is required by the MapBox-gl library we are using.

It also is difficult to figure out how to package the tiles in a way that makes sense, and requires significant effort for first-time setup.

Here are the docs currently for how to do this, which are somewhat incomplete: https://docs.mapeo.app/customization/aerial-imagery

[okdistribute]

cc @gmaclennan @arky this is requested in the field (also for mobile) and is on the upcoming roadmap. However, still figuring out our timing about when we will be able to ship this feature