Support associating release notes with one or more countries

[stevejalim]

Nucleus-side work towards #827

Adds a Country model, backfills it with data and adds a relevant_countries field to Note model

The representation of a Note in the all-releases.json now features a relevant_countries key, which has a list of dicts:

     {
        "id": 7,
        "bug": null,
        "note": "Elit id consequat, occaecat cillum. Occaecat cillum, ipsum quis. Quis laboris, dolor amet nisi aliquip velit consequat. Amet nisi aliquip velit consequat officia ipsum duis. Aliquip velit consequat officia ipsum, duis id amet.",
        "tag": "",
        "sort_num": 0,
        "is_public": true,
        "progressive_rollout": false,
        "relevant_countries": [
          {
            "name": "Angola",
            "code": "AO"
          },
          {
            "name": "Anguilla",
            "code": "AI"
          },
          {
            "name": "Antarctica",
            "code": "AQ"
          }
        ],
        "created": "2023-11-23T15:20:58.814643+00:00",
        "modified": "2023-11-23T16:03:18.389135+00:00"
      }

And this is present but empty if there are no specifically-associated countries:

        "relevant_countries": [],

--

There will be complementary work on the Bedrock side to read in this data

[robhudson]

All looks good to me

[stevejalim]

For the record, mozilla/bedrock#13931 is the mozorg-side code change to show this data