Bump Slimmer to 3.27.0

This gives us the new beta label processor
Remove old beta label styles
Use the new beta label provided by Slimmer

Set the correct headers on specialist sector browse pages so that
Slimmer will insert the beta header after the sector title.

There's a small bit of CSS included in browse.scss to set margins to
that the new notice is positioned in the same place as the previous one.
Merge pull request #542 from alphagov/slimmered-beta-label

Replace the specialist browse beta label with the one from Slimmer

https://www.pivotaltracker.com/story/show/65975438