reduce amount of magic values, goal is to have specific states -> e.g. cancelled

src/model/tripdetails.rs

@@ -95,7 +95,7 @@ WHERE day = ? AND planned_starting_time = ?
     }
 
     pub fn cancelled(&self) -> bool {
-        self.max_people == 0
+        self.max_people == -1
     }
 
     /// This function is called when a person registers to a trip or when the cox changes the