meta: introduce Cypress end-to-end testing (via #4827)

* `test/e2e` -> `test/e2e-selenium`

* add Cypress

* ESLint fixes

* MOAR cypress

* `integration` -> `tests`

* wire Cypress up to a hot e2e server

* linter fixes

* don't run in CI

test/e2e-cypress/support/commands.js

@@ -0,0 +1,25 @@
+// ***********************************************
+// This example commands.js shows you how to
+// create various custom commands and overwrite
+// existing commands.
+//
+// For more comprehensive examples of custom
+// commands please read more here:
+// https://on.cypress.io/custom-commands
+// ***********************************************
+//
+//
+// -- This is a parent command --
+// Cypress.Commands.add("login", (email, password) => { ... })
+//
+//
+// -- This is a child command --
+// Cypress.Commands.add("drag", { prevSubject: 'element'}, (subject, options) => { ... })
+//
+//
+// -- This is a dual command --
+// Cypress.Commands.add("dismiss", { prevSubject: 'optional'}, (subject, options) => { ... })
+//
+//
+// -- This is will overwrite an existing command --
+// Cypress.Commands.overwrite("visit", (originalFn, url, options) => { ... })