Implemented the second round of the toResponse normalisationf for all classes

ref #67

src/models/entities/FixedDonation.ts

@@ -16,4 +16,11 @@ export class FixedDonation extends Donation {
   @IsInt()
   @IsPositive()
   amount: number;
+
+  /**
+   * Turns this entity into it's response class.
+   */
+  public toResponse() {
+    return new Error("NotImplemented");
+  }
 }