feat: add public and school holidays for UK

[kassi]

This PR adds UK, the United Kingdom, to the database.

The official ISO code is GB with official name "(the) United Kingdom of Great Britain and Northern Ireland".
The ISO code UK is reserved by the United Kingdom such that no country will every use it, however it's not the official one to use, so I used GB.

I used the name United Kingdom (Vereinigtes Königreich) here for convenience, because Great Britain actually only consists of England, Scotland and Wales, excluding Northern Ireland.

The Scots language doesn't have a 639-1 ISO code, only 639-2 (3 letters), so I used that.

The information about public holidays is fetched from https://www.gov.uk/bank-holidays
The information about school holidays is fetched from

• https://www.education-ni.gov.uk/articles/school-holidays
• https://www.gov.scot/policies/schools/school-term-dates
• https://www.gov.wales/school-term-dates
• https://www.gov.uk/school-term-holiday-dates, but needed to be extracted for each authority.

WIP: I'm on it to re-verify the individual authorities' schedules, looking for alternative sources and/or creating a tool to check it (which may then be used on a regular base from year to year)

[fstueber]

Wow, great. Thank you! I'll have a look at this over the next few days. Just in case: UK bank holdiays can be accessed via API: https://www.gov.uk/bank-holidays.json

[kassi]

Good to know. Makes things easier. Would be great if they had the same for school holidays. Let's see what they say.
Remember, it's still WIP. I think I can take a deeper look and continue within the next 2 weeks. Let's see.