Make aerial imagery offline

[rusty-snake]

Use case
Copied from #742:

When solving quests and adding new data, it's incredibly valuable to have background imagery enabled. It helps users ensure they are matching the map feature to the correct real world feature, and it allows new data to be placed more accurately.

Proposed Solution

aerial imagery is currently cached (but the cache is to small) when you view it.
There should be an download button to download all tiles¹ for a area or all tiles¹ for the areas that have downloaded OSM data (like in Every Door).
All tiles for a specific zoom level (e.g. 19) would be enough.

[andrewharvey]

I think this will be useful for users in areas of no coverage or users who do not have data or do not wish to use a mobile network.