Convert sticker end-to-end tests to Cypress (#8807)

* Convert sticker end-to-end tests to Cypress

Reference materials:
* https://www.cypress.io/blog/2020/02/12/working-with-iframes-in-cypress/
* https://github.com/cypress-io/cypress/issues/136
* https://docs.cypress.io/api/commands/origin#Other-limitations

Ideally we'd be able to use `cy.origin()` to jump into the iframe, but it's explicitly not supported. Instead we disable web security as instructed by cypress because it's our only reasonable option here. Thankfully, disabling web security doesn't appear to remove the crypto libraries from the browser so we can still function in that respect.

Rationale for why we can't just serve the sticker picker off the app domain is included in the code.

* Appease the linter

* More linter appeasement

cypress/support/iframes.ts

@@ -0,0 +1,45 @@
+/*
+Copyright 2022 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/// <reference types="cypress" />
+
+import Chainable = Cypress.Chainable;
+
+declare global {
+    // eslint-disable-next-line @typescript-eslint/no-namespace
+    namespace Cypress {
+        interface Chainable {
+            /**
+             * Gets you into the `body` of the selectable iframe. Best to call
+             * `within({}, () => { ... })` on the returned Chainable to access
+             * further elements.
+             * @param selector The jquery selector to find the frame with.
+             */
+            accessIframe(selector: string): Chainable<JQuery<HTMLElement>>;
+        }
+    }
+}
+
+// Inspired by https://www.cypress.io/blog/2020/02/12/working-with-iframes-in-cypress/
+Cypress.Commands.add("accessIframe", (selector: string): Chainable<JQuery<HTMLElement>> => {
+    return cy.get(selector)
+        .its("0.contentDocument.body").should("not.be.empty")
+        // Cypress loses types in the mess of wrapping, so force cast
+        .then(cy.wrap) as Chainable<JQuery<HTMLElement>>;
+});
+
+// Needed to make this file a module
+export { };