Show trip total distance on trip list page

Closes: #142

agenda/types.py

@@ -133,6 +133,14 @@ class Trip:
         """Countries flags for trip."""
         return "".join(c.flag for c in self.countries)
 
+    def total_distance(self) -> float | None:
+        """Total distance for trip."""
+        return (
+            sum(t["distance"] for t in self.travel)
+            if all(t.get("distance") for t in self.travel)
+            else None
+        )
+
 
 @dataclass
 class Holiday: