added e2e tests (#298)

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

cypress/integration/language.spec.ts

@@ -0,0 +1,30 @@
+import { languages } from '../fixtures/languages'
+
+describe('Languages', () => {
+  beforeEach(() => {
+    cy.visit('/')
+  })
+
+  it('all languages are available', () => {
+    cy.get('option')
+      .as('languages')
+    cy.get('@languages')
+      .should('have.length', 28)
+    languages.forEach(language => {
+      cy.get('@languages').contains(language)
+    })
+  })
+
+  it('language changes affect the UI', () => {
+    cy.get('select')
+      .select('English')
+    cy.get('.d-inline-flex.btn-primary')
+      .find('span')
+      .contains('New note')
+    cy.get('select')
+      .select('Deutsch')
+    cy.get('.d-inline-flex.btn-primary')
+      .find('span')
+      .contains('Neue Notiz')
+  })
+})