Add European 6 channel roaming plan

[johanstokking]

Summary

Closes #8

This is according to the "Best Practices for European Operators" version 3. This hasn't been fully ratified by the Technical Committee yet, mostly because of editorial work and IP review. Therefore, this one's _DRAFT.

Supported by Orange, Swisscom, Bouygues, CRA, Netmore, Digita, KPN, Proximus and The Things Network.

Changes

Add new EU_863_870_ROAMING_DRAFT frequency plan with the six common channels used by the participating operators.

Notes for Reviewers

We're using SF12 here only, as that is what all the other operators use for RX2. I'm afraid this is what they optimize their networks for, so let's not bother with a SF9 variant.

Checklist

• [x] Scope: The referenced issue is addressed, there are no unrelated changes.
• [x] Compatibility: The changes are backwards compatible, they don't break existing deployments.
• [x] Testing: The changes are tested.
• [x] Documentation: Relevant documentation is added or updated.

[johanstokking]

Example join-accept from TTS:

    "join_accept_payload": {
      "encrypted": "9ObehGbfWGjg0bjAR/maE0SLcT9hpEn4S/Ju4XqKT+o=",
      "join_nonce": "000001",
      "net_id": "000000",
      "dev_addr": "00000001",
      "dl_settings": {

      },
      "rx_delay": 5,
      "cf_list": {
        "freq": [
          8671000,
          8673000,
          8679000
        ]
      }
    }