Added the donation classes

ref #1

src/models/FixedDonation.ts

@@ -0,0 +1,16 @@
+import { IsInt, IsPositive } from "class-validator";
+import { Donation } from "./Donation";
+
+/**
+ * Defines the FixedDonation entity.
+ * In the past there was no easy way to track fixed donations (eg. for creating donation receipts).
+*/
+export class FixedDonation extends Donation {
+
+  /**
+   * The donation's amount in cents (or whatever your currency's smallest unit is.).
+   */
+  @IsInt()
+  @IsPositive()
+  amount: number;
+}