Use map tiles dark mode with leaflet-osm plugin

[hlfan]

This lean and updated variant of #4777 follows the rules laid out in #5328 that were implemented in #5362.
Since the editing of the user preference happens on a different view/site, there is no need to listen to color scheme changes on the fly and the whole logic can happen in the initialization of L.OSM.TileLayer.

Before	After

Additionally, a "dark" CSS class is introduced to allow better style targeting if the user wants to.
for example like this:

[tomhughes]

Changes to anything under vendor need to be done in the appropriate upstream repository, not here.

[tomhughes]

Also inline styles are not allowed by our security policy in production so any dynamic changes need to be done by adding or removing classes to trigger static style rules in the main stylesheet.

[hlfan]

I wrote it's a leaner variant of #4777, not that I fixed its flaws.

Changes to anything under vendor need to be done in the appropriate upstream repository, not here.

That's why I left openstreetmap/leaflet-osm#42 open.

inline styles are not allowed by our security policy in production

The style-src 'self' 'unsafe-inline' in the CSP header tells me a different story.

[tomhughes]

I don't think leaflet-osm should be getting involved in automatic transforms - it should have the ability to select a dark tile version where there is one and any automatic transformations should only be in the osm.org code.

[tomhughes]

There is no lightUrl anywhere?

[hlfan]

Well, if the provided layers can be expected always to be a light-mode-first choice, the logic can be simplified to this more unreadable version:

    options.filter = isDarkMap && options.darkFilter || 'none';
    options = L.Util.setOptions(this, options);
    url = isDarkMap && options.darkUrl;
    this.schemeClass = url && 'dark';

But I thought an implementation agnostic to that would be easier to think through with more options for cartographers' choices.

[tomhughes]

inline styles are not allowed by our security policy in production

The style-src 'self' 'unsafe-inline' in the CSP header tells me a different story.

OK so it is allowed for views with a map. I'm note sure why that is offhand but our goal is to not have it so we don't add new inline styles ourselves but we sometimes have to allow it for third party components.

[hlfan]

leaflet-osm should be getting involved in automatic transforms - it should have the ability to select a dark tile version where there is one

Do you mean only the extension of L.OSM.TransportMap should be added to leaflet-osm?