Offline map tile caching

[mstenta]

We are going to be incorporating farmOS-map into the farmOS-client, which is an offline-first progressive web app. One of the limitations that maps currently have is that tiles are loaded over the network, so creating an OpenLayers map without internet connection doesn't work.

Ideally, the farmOS-map library would include functionality to cache tiles locally so that it could work offline. Or, maybe this is best left to applications like farmOS-client? Let's discuss!