Show a geodesic line from LHR to the conference venue if no travel booked

Closes: #111

agenda/trip.py

@@ -191,7 +191,21 @@ def get_trip_routes(trip: Trip) -> list[StrDict]:
                 }
             )
 
-    return routes
+    if routes:
+        return routes
+
+    lhr = (51.4775, -0.461389)
+
+    return [
+        {
+            "type": "flight",
+            "key": f'LHR_{item["location"]}_{item["country"]}',
+            "from": lhr,
+            "to": latlon_tuple(item),
+        }
+        for item in trip.conferences
+        if "latitude" in item and "longitude" in item and item["country"] != "gb"
+    ]
 
 
 def get_coordinates_and_routes(