Map color mode preference

[AntonKhorev]

Includes #5339. Implements step 5 from #5324 (comment).

[hlfan]

Doesn't the mentioned step 5 include the need for the filters to be set individually to give the cartographers a choice?
I don't see that as a necessity for this PR, but already setting the filter through a custom property defined on the layer could ease the transition to that being implemented and also act as default and fallback:

here {
    filter: var(--dark-mode-map-filter);
}
somewhere else {
    .leaflet-layer {
        --dark-mode-map-filter: brightness(.8);
    }
}

[AntonKhorev]

Step 5 has an impossible requirement of "make it dark but don't pick any filter". The closest possible thing is "don't touch existing filters".

[omcnoe]

This will only be configurable for logged in users, meaning that non-logged in will still be stuck on the dark UI + dimmed tiles.

[gravitystorm]

Thanks @AntonKhorev , I think this works well.