added e2e tests (#298)

- added e2e tests for
  - banner
  - history
  - intro
  - language
  - link
- added e2e workflow
- added cypress badge to README

cypress/support/login.ts

@@ -0,0 +1,15 @@
+// eslint-disable-next-line @typescript-eslint/no-namespace
+declare namespace Cypress {
+  interface Chainable {
+    /**
+     * Custom command to log the user out.
+     * @example cy.logout()
+     */
+    logout (): Chainable<Window>
+  }
+}
+
+Cypress.Commands.add('logout', () => {
+  cy.get('#dropdown-user').click()
+  cy.get('.fa-sign-out').click()
+})