Added finxed donations

ref #11

src/models/FixedDonation.ts

@@ -0,0 +1,18 @@
+import { Entity, Column } from "typeorm";
+import { IsInt, IsPositive,} from "class-validator";
+import { Donation } from "./Donation";
+
+/**
+ * Defines a fixed donation.
+*/
+@Entity()
+export class FixedDonation extends Donation {
+
+  /**
+   * The donation's amount in cents (or whatever your currency's smallest unit is.).
+   */
+  @Column()
+  @IsInt()
+  @IsPositive()
+  amount: number;
+}