Stop relying on external resources

[relrod]

Eventually (especially once #2 lands and we can do this completely), we'll want to stop requiring an active internet connection to use this (especially in chase car mode -- cf. #8).

This means:

• Copying all JS/CSS that we use, and
• Letting people run their own local tile server.

I think 2) shouldn't be bad, but 1) is blocked until we can stop depending on gmaps.

Once we do this, it should be possible, in theory, to run mapview(-psc) entirely without an internet connection, which will be useful if we ever have to chase a balloon into the mountains of WPA where there is likely no cell network.