Remove slimmer search-index-header

It is no longer being used as there is no longer a tabbed search.
Bump slimmer to 4.0.0

For a version which doesn't have search-index header. Also in this
version multiple need ids, new beta label inserter and analytics
updates.
Merge pull request #1488 from alphagov/remove-search-index-header

Remove search index header slimmer header