Booking Creation - Finish initial implement.

- Check and update availability.
- Set prices.
- Generate booking refNumber.

src/Entity/RoomAvailability.php

@@ -57,4 +57,8 @@ class RoomAvailability
 
         return $this;
     }
+
+    public function modNumAvailable(int $diffAmount):void {
+        $this->numAvailable -= $diffAmount;
+    }
 }