Feature/add party nations

[Bekabyx]

Trello

Peter requested that we add a breakdown of which nations parties field candidates in - this data is part of the EC's party dataset (see here for example, "Fielding Candidates in England" etc.) and is available in their API.

Sym recommended that rather than storing these three fields separately we combine the data into a single field so that's the approach I've taken.

To test locally:

• Run manage.py migrate
• Run manage.py parties_import_from_ec
• Run manage.py runserver
• Find non-joint parties on the GB register and the NI register, one each (In the next steps I've chosen specific parties but change it up if you like)
• Hit your local YNR API for the GB party, e.g.http://localhost:8000/api/next/parties/PP53/ and check the nations field - this should be populated e.g. ["ENG","WAL","SCO"].
  • The nations in the list should match whatever is on the EC register for that party under the three "Fielding candidates in x" fields.
  • EC register link e.g. https://search.electoralcommission.org.uk/English/Registrations/PP53
• Hit your local YNR API again for the NI party (e.g. PP51) and check the nations field - this should be null

Blocks DemocracyClub/WhoCanIVoteFor#1655

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