announce neighbours using alfred/gluon-announce

This adds a new announce.d datum "neighbours" (alfred 160) containing
information about mesh neighbours. It's intended to be an replacement
for batadv-vis.

In addition to the data already provided by batadv-vis it'll also
provide information about direct wifi neighbours.

Unlike batadv-vis, no data about clients is transmitted.

Sample data:

    {
      "wifi": {
        "90:f6:52:82:06:02": {
          "neighbours": {
            "f8:d1:11:2c:a7:d2": {
              "noise": -95,
              "inactive": 0,
              "signal": 0
            },
            "96:f6:52:ff:cd:6f": {
              "noise": -95,
              "inactive": 0,
              "signal": -37
            }
          }
        }
      },
      "batadv": {
        "90:f6:52:82:06:02": {
          "neighbours": {
            "96:f6:52:ff:cd:6f": {
              "lastseen": 2.8500000000000001,
              "tq": 177
            }
          }
        },
        "90:f6:52:82:06:03": {
          "neighbours": {
            "f8:d1:11:2c:a7:d3": {
              "lastseen": 2.3500000000000001,
              "tq": 206
            }
          }
        }
      },
      "node_id": "90f652820602"
    }