Move hiding logic to stimulus controller

This ensures morphed flashes hide like other flashes (eg in bulk order actions). I wanted to write a spec to prove it, but Capybara doesn't support mocking setTimeout and I didn't want to use sleep.

I've made it optional because this controller is shared with the shop frontend ([supposedly](5ef34347a3), although angular seems to override it).

spec/javascripts/stimulus/flash_controller_test.js

@@ -0,0 +1,34 @@
+/**
+ * @jest-environment jsdom
+ */
+
+import { Application } from "stimulus";
+import flash_controller from "../../../app/webpacker/controllers/flash_controller";
+
+describe("FlashController", () => {
+  beforeAll(() => {
+    const application = Application.start();
+    application.register("flash", flash_controller);
+  });
+
+  beforeEach(() => {
+    document.body.innerHTML = `
+      <div id="element" data-controller='flash' data-flash-auto-close-value='true'></div>
+    `;
+
+  });
+
+  describe("autoClose", () => {
+    jest.useFakeTimers();
+
+    it("is cleared after about 5 seconds", () => {
+      let element = document.getElementById("element");
+      expect(element).not.toBe(null);
+
+      jest.advanceTimersByTime(5500);
+
+      element = document.getElementById("element");
+      expect(element).toBe(null);
+    });
+  });
+});