Add somerset unitary authority

[GeoWill]

I'm making this PR because Democracy Club needs to create organisations to hold elections in EveryElection. It would be good if we can keep the local-authority-code values in sync, so this is to figure out what that value will be. As long as the code is settled on then we are in no hurry for this to be merged.

• Draft legislation: https://www.legislation.gov.uk/ukdsi/2022/9780348231366
• We don't have a GSS code yet
• end-dates - Bucks districts have an end date that is the day before the start date of Bucks UA. North Northants uses the same day. I've used the same day because that's what it says in the legislation, but happy to change.

[GeoWill]

Actual legislation: https://www.legislation.gov.uk/uksi/2022/329/made

This isn't going to be made in EE till next year, because afaict the elections this year are to the county council, just using the same divisions as will exist in the unitary.