Add offline map/tile support

[davecraig]

This would add downloading of a geographical region into the phone which can then be used on the phone without access to a network. There are currently two sets of data:

1. The UI map tile data
2. The soundscape-backend provided GeoJSON tiles used for audio callouts

Both are currently cached on the phone, but the caches have fixed (and arbitrary) sizes and we want to be able to download a small region and keep it stored for a longer period of time. Note that in the future (#152) we are hoping to move to just using the UI map tile data which would mean a single source of data.

The UI could be something as simple as "save all tiles within 5 miles)" or it could involve drawing a rectangle on a map to select the region to save. The UI would also have to allow "refreshing" of the region to get any mapping updates and "deleting" of the region. Multiple regions would likely also be useful. Most mapping apps have a similar feature e.g. Komoot, OS Maps, Google maps.