Import cookie banner macro

Import the cookie banner so that users can include it in the prototype kit without
needing to import it themselves, as we tell them to do in the ‘Getting Started’
documentation in the Design System[1]

[1]: design-system.service.gov.uk/get-started/prototyping/#:~:text=When%20using%20Nunjucks%20macros%20in%20the%20Prototype%20Kit%20leave%20out%20the%20first%20line%20that%20starts%20with%20%7B%25%20from%20....

app/views/layout.html

@@ -9,6 +9,7 @@
 {% from "govuk/components/button/macro.njk"              import govukButton %}
 {% from "govuk/components/character-count/macro.njk"     import govukCharacterCount %}
 {% from "govuk/components/checkboxes/macro.njk"          import govukCheckboxes %}
+{% from "govuk/components/cookie-banner/macro.njk"       import govukCookieBanner %}
 {% from "govuk/components/date-input/macro.njk"          import govukDateInput %}
 {% from "govuk/components/details/macro.njk"             import govukDetails %}
 {% from "govuk/components/error-message/macro.njk"       import govukErrorMessage %}