Expose generic aggregations in API

[jjst]

Currently:

GET /stations/<country_code>/<station_id>/now-playing

Replace with:

GET /stations/<country_code>/<station_id>/<aggregation>

Where 'now-playing' becomes one type of aggregation.

The parent endpoint:

GET /stations/<country_code>/<station_id>

now includes the list of aggregations a station supports, e.g. for fr/radiomeuh/:

"aggregations": {
  "now-playing" : {
    "href": "https://hostname/api/stations/fr/radiomeuh/now-playing"
  }
}