introduce tsmap placeholders distancem and distancekm

[wrohdewald]

I did not change translation strings. And I am not sure about miles. distancem clearly is the distance in meters, but what is the correct name for miles?

Please not that the help text did not yet mention {distance} at all, maybe {distance} can be removed. {speedX} placeholders always include the unit in their name, so maybe it should be the same with {distanceX}

[tinuzz]

I'd say 'distancemi'. I'll add this!

[tinuzz]

Also, I think it's not necessary to add new metadata in the server response. These values can be derived from the distance in meters, so they can just be calculated in JS.