store full rtsp urls

My dad's "GW-GW4089IP" cameras use separate ports for the main and sub
streams:

rtsp://192.168.1.110:5050/H264?channel=0&subtype=0&unicast=true&proto=Onvif
rtsp://192.168.1.110:5049/H264?channel=0&subtype=1&unicast=true&proto=Onvif

Previously I could get one of the streams to work by including :5050 or
:5049 in the host field of the camera. But not both. Now make the
camera's host field reflect the ONVIF port (which is also non-standard
on these cameras, :85). It's not directly used yet but probably will be
sooner or later. Make each stream know its full URL.

design/api.md

@@ -76,7 +76,7 @@ The `application/json` response will have a dict as follows:
         a dictionary describing the configuration of the camera:
         *   `username`
         *   `password`
-        *   `host`
+        *   `onvif_host`
     *   `streams`: a dict of stream type ("main" or "sub") to a dictionary
         describing the stream:
         *   `retainBytes`: the configured total number of bytes of completed
@@ -136,7 +136,7 @@ Example response:
       "shortName": "driveway",
       "description": "Hikvision DS-2CD2032 overlooking the driveway from east",
       "config": {
-        "host": "192.168.1.100",
+        "onvif_host": "192.168.1.100",
         "user": "admin",
         "password": "12345",
       },