[Maps] set assets/js/leaflet/leafembed.js hardcoded maps tile server to use the options map_tile_server address

[magicbug]

The js file assets/js/leaflet/leafembed.js uses a hardcoded map tile URL, this needs to be changed to use the options database map_tile_server field instead so it can be changed on the fly.

Thanks to @samhjn for pointing this out.