Don't display down status for inactive receivers

[pyrog]

Status "Up" or "Down" from APRSC server is precious for receivers in "production".
But for inactive receivers, the "Down" status is a bit confusing.

Current request to OGNRange doesn't return any data for them.

To get the last date of activity (during the last 2 years), we could use Grafana datas or OGN Range stats.

Grafana

• Send POST request to http://grafana.glidernet.org/api/datasources/proxy/1/api/query
  Requested data: CPU from 01 Jan 2000 00:00:00 for AireVille APRS CallSign

{
  "start": 946684800,
  "queries":[
    {
      "metric":"cpu",
      "aggregator":"max",
      "downsample":"12h-max",
      "tags":{"receiver":"AireVille"}
    }
  ],
  "msResolution":false,
  "globalAnnotations":true
}

• Receive

[
  {
    "aggregateTags": [], 
    "dps": {
      "1439629067": 0.7, 
…
      "1462438667": 0.1
    }, 
    "metric": "cpu", 
    "tags": {
      "receiver": "AireVille"
    }
  }
]

AireVille receiver was active from 15 Aug 2015 08:57:47 to 05 May 2016 08:57:47

OGN Range

• Send GET request to https://ognrange.onglide.com/api/1/stats?station=AireVille&start=2000-01-01
• Receive

{
  "stats": [
    {"c": 0, "g": 0, "la": 0.0, "p": 0, "t": "2016-05-04 18:35", "temp": 0}, 
…
    {"c": 0, "g": 0, "la": 0.0, "p": 0, "t": "2016-05-04 05:05", "temp": 0}
  ]
}

AireVille receiver was active to 04 May 2016 05:05:00