Add weekend styling option

[ubhaller]

Add date select option to apply style to weekend days

[mikaelmello]

If I understand it right, this isn't quite an option but we're making weekends styled with red.

I feel the red conveys a quite different user experience than the existing model.

I went to check how Google Flights and Linear do it (they're like my references for the date picker lol) and Google does not highlight weekends at all, while Linear does it with the same light grey used to represent dates in the prev/next months.

I think this should be grey by default instead of red, and it should also be a configurable option (would it be on or off by default?)