Update to latest version of NHS.UK frontend (4.0.0)

[chrimesdev]

Description

NHS.UK frontend 4.0.0 was released in October, we need to look at updating the Wagtail frontend to use the latest version

To do

• [x] Update to v4.0.0 CSS and JavaScript
• [x] Remove Panel and Promo components to be replaced with the Card component (@pflynny)
• [ ] Add Tag component (@AdamChrimes)
• [ ] Add custom search API endpoint to the example app
• [x] Look at the Warning callout default "Important" prefix