[Feature] Add support for specific UK countries

[SpedNSFW]

Is your feature request related to a problem? Please describe.
When adding nationalities to performers, instead of being able to select a specific member country of the United Kingdom (England, Scotland, Wales, Northern Ireland), you can only select the UK. This means we are missing precision. While the UK is a country, so to are the four member states.

Describe the solution you'd like
Without removing the United Kingdom as an option, add the member states: England, Scotland, Wales, and Northern Ireland.
A matching change will need to be made in Stash to support these options with flags added for each.

There is no ISO 3166 code for each country, but the codes around the UK already don't make sense (the code is GB, which would exclude Northern Ireland). Because of this, we'd need to add exceptions for these four.

Describe alternatives you've considered
I don't see any alternate option for this. Either we support the specific countries, or we don't. Not supporting them is frankly a bad option for a database that aims to be correct and specific where possible.

Additional context
The way I see this working is when a performer's nationality is known (either through an obvious accent or the city they're from is known), we can set it to the specific country, otherwise it would stay as the UK. For example, we know Ella Hughes is from Southhamption, so she could be set to England.

[SpedNSFW]

I suggest it because there are many relatively recent countries no longer supported (listed here) where a performer could have been born.

That's definitely true and not something I entirely considered when I made this issue. There are going to be people born in Czechoslovakia, Yugoslavia, and the USSR.

While supporting a custom list like that would work for the local install, it may not be the best option if we wanted to keep things in sync with stash-box. It might just be simpler to add the member states of the UK to both to start with.

Adding dissolved/renamed countries may receive backlash as it's an intrinsically political issue, though the codes do exist as per ISO 3166-3 as you've linked.

[S3L3CT3DLoves]

Been thinking about your issue for a while. And the more I think about it, the more I think it would end up creating an unpleasant mess if we simply "add" the member states in the list.
Mostly because, if we do it for the UK, why would we not do it for the US? Germany? Australia? India? ... There are a lot of federates states in the world.

The only way I would see this work is with a second field for the performer, which determines the subdivision. (Otherwise the UI will be unusable)

ISO 3166-2 keeps a list of the subdivision codes for each ISO 3166 country. Examples: UK US

But that also means regions if the country isn't a federated state, for example Ukraine UA , and somes cities, which are often independent subdivisions in countries, for example Zurich in Switzerland CH.
So how do we keep it at a reasonable level of granuarity?

I have two ideas in mind:

1. Tags - Easy, DIY, already available, not perfect but it works
2. Use ISO 3166-2 in a new dedicated field, for a specific list of countries defined by the Dev/Admin -- This would also need to be replicated in Stash

Separately, I also agree that the Nationality field needs to become multi-select, many performers have more than one nationality. But that should be a separate issue imo, because including ISO 3166-2 codes in the main "country list" would really make that list extremely long.