Sick Days

[coijanovic]

Same handling as vacation days, but should be separate for more granulated statistics.

Implement once issue #3 is closed as partial sick days are also needed anyways.

[Kingdread]

Maybe we should just call them "skip days" (or "ignored days")? I'd also like to add travel/excursion/... days to be ignored without them being counted as vacation, so it would coincide with the handling of "sick days".