Add utility functions to set and delete cookies

Issue: GREG-89

frontend/src/utils/index.test.ts

 import {
   getCookie,
+  deleteCookie,
+  setCookie,
   isValidEmail,
   isValidFnr,
   isValidMobilePhoneNumber,
@@ -77,6 +79,33 @@ test('Get unknown cookie returns null', async () => {
   document.cookie = 'csrftoken= ; expires = Thu, 01 Jan 1970 00:00:00 GMT'
 })
 
+test('Get known cookie returns value', async () => {
+  document.cookie = 'key=value'
+  expect(getCookie('key')).toEqual('value')
+  document.cookie = 'key= ; expires = Thu, 01, Jan 1970 00:00:00 GMT'
+})
+
+test('Deleting cookie removes it', async () => {
+  document.cookie = 'key=value'
+  expect(getCookie('key')).toEqual('value')
+  deleteCookie('key')
+  expect(getCookie('key')).toEqual(null)
+})
+
+test('setCookie creates a cookie with correct value', async () => {
+  setCookie('key', 'value')
+  expect(getCookie('key')).toEqual('value')
+  deleteCookie('key')
+})
+
+test('setCookie overrides value that was set before', async () => {
+  setCookie('key', 'value')
+  expect(getCookie('key')).toEqual('value')
+  setCookie('key', 'differentvalue')
+  expect(getCookie('key')).toEqual('differentvalue')
+  deleteCookie('key')
+})
+
 test('Valid fnr', async () => {
   expect(isValidFnr('04026514903')).toEqual(true)
 })

frontend/src/utils/index.ts

@@ -23,6 +23,16 @@ export function getCookie(name: string) {
   return decodeURIComponent(cookies[0].split('=')[1])
 }
 
+export function setCookie(name: string, value: string) {
+  document.cookie = `${name}=${value}; path=/`
+}
+
+export function deleteCookie(name: string) {
+  if (getCookie(name)) {
+    setCookie(name, '; expires=Thu, 01 jan 1970 00:00:00 GMT')
+  }
+}
+
 export function maybeCsrfToken() {
   const csrfToken = getCookie('csrftoken')
   if (!csrfToken) {