Implemented the second round of the toResponse normalisationf for all classes

ref #67

src/models/entities/DistanceDonation.ts

@@ -39,4 +39,11 @@ export class DistanceDonation extends Donation {
     }
     return calculatedAmount;
   }
+
+  /**
+   * Turns this entity into it's response class.
+   */
+  public toResponse() {
+    return new Error("NotImplemented");
+  }
 }