Use transport-dark tiles for Transport map in dark mode

[gravitystorm]

See #5328 for background.

As the cartographer for the transport featured layer, I would like this project to use the transport-dark variant when our maps are shown in dark mode.

[hlfan]

Should the OpenCycleMap layer stay dimmed with reduced contrast as it currently is or should another filter (if any) be chosen to optimize the map for dark mode?

[gravitystorm]

Should the OpenCycleMap layer stay dimmed with reduced contrast as it currently is or should another filter (if any) be chosen to optimize the map for dark mode?

I'd prefer to see the OpenCycleMap layer displayed normally in dark mode (i.e. with no filter) for now. I think either we will find a different filter, or more likely, create a "dark" variant instead.