[openstreetmap/openstreetmap-website] Introduce Dark Mode (WIP) (#3028)

[pkrasicki]

@bhousel I did the same in https://issviewer.com.
![dark-theme-toggle](https://user-images.githubusercontent.com/49721209/103914868-57d08580-5102-11eb-8210-9da577c39293.png)

Seems like a reasonable alternative to me.

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