Add GOVUK Frontend version of govuk_template
Add GOVUK Frontend version of govuk_template

`admin_template.html` is the base layout used by
all pages in the application.

This swaps out its dependance on govuk_template
for the version GOVUK Frontend provides.

Also renames blocks used in `admin_template.html`,
defined in both the old and new govuk_template
whose names have changed, as listed in:

https://design-system.service.gov.uk/styles/page-template/

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin