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

cypress/support/index.ts

@@ -16,6 +16,8 @@ limitations under the License.
 
 /// <reference types="cypress" />
 
+import "@percy/cypress";
+
 import "./performance";
 import "./synapse";
 import "./login";