Add visual tests using Percy in Cypress (#8610)

* Add basic Percy tests

* Run Percy in CI

* Fix snapshot timing

* Fix bad selector

* Hide another bit of dynamic text

* Add docs

package.json

@@ -133,6 +133,8 @@
     "@babel/traverse": "^7.12.12",
     "@matrix-org/olm": "https://gitlab.matrix.org/api/v4/projects/27/packages/npm/@matrix-org/olm/-/@matrix-org/olm-3.2.8.tgz",
     "@peculiar/webcrypto": "^1.1.4",
+    "@percy/cli": "^1.1.4",
+    "@percy/cypress": "^3.1.1",
     "@sentry/types": "^6.10.0",
     "@sinonjs/fake-timers": "^9.1.2",
     "@types/classnames": "^2.2.11",