Color mode preference stored in db

[AntonKhorev]

The same as #5337 but stores the preferred color scheme value in user preferences db table instead of cookies.

[gravitystorm]

I think the preference should have three options - auto (default), light and dark. In auto mode, either the browser color-mode should be used, or if that's not available, we should use light mode.

See #5324 (comment) for more details.

[AntonKhorev]

I also think that it should have three options. The question is do you want a larger pull request that adds three options or do you want a smaller one (you usually say you want small pull requests) that lets you disable the auto dark mode?

[gravitystorm]

I think if we both agree to add a new preference with 3 options, there's no need to split that into different pull requests for each option! 😄

[AntonKhorev]

Updated with dark option