Reply to originally invoking message for reminders

We only do this if we have a reminder body, otherwise we would link to the original message twice.

backend/src/migrations/1631494429170-AddSourceMessageIdToReminders.ts

@@ -0,0 +1,19 @@
+import { MigrationInterface, QueryRunner, TableColumn } from "typeorm";
+
+export class AddSourceMessageIdToReminders1631494429170 implements MigrationInterface {
+  public async up(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.addColumns("reminders", [
+      new TableColumn({
+        name: "source_message_id",
+        type: "bigint",
+        isNullable: true,
+        default: null,
+        unsigned: true,
+      }),
+    ]);
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.dropColumn("reminders", "source_message_id");
+  }
+}