feat: per-year holidays

[NecroKote]

Hi! It's me again :)

Today I bring you the "preview" of Holidays handling. (#266)

PR contains:

• new Holidays Settings item
• ability to add and remove Holidays for given year
• ability to load country holidays from date.nager.at for given date (using 2 letter country code)
• holidays are excluded during vacation and sick day calculations
• holidays are displayed as vacations on calendar views
• small bits and bobs - reusable components, small refactors and optimisations here and there

It's not a polished PR by any means, I just really wanted to get it to work.

Known issues:

• 🐛 design flaws around holidays in general
• 🐛 complete lack of user guidance around the feature
• 🐛 no logic for handling collisions when loading holidays, e.g perform load twice - get twice the holidays! 🎉

Any feedback is appreciated!

[CrazyMarvin]

Hey @NecroKote,
thanks again for an awesome PR! 👍