Sign webhook requests (#4156)

Co-authored-by: Tom Moor <tom.moor@gmail.com>

server/migrations/20220922073737-webhook-signing-secret.js

@@ -0,0 +1,14 @@
+"use strict";
+
+module.exports = {
+  async up (queryInterface, Sequelize) {
+    return queryInterface.addColumn("webhook_subscriptions", "secret", {
+      type: Sequelize.BLOB,
+      allowNull: true,
+    });
+  },
+
+  async down (queryInterface, Sequelize) {
+    return queryInterface.removeColumn("webhook_subscriptions", "secret");
+  }
+};