Change recommending passphrase_entropy to zxcvbn

As @edwardhorsford noted in Campfire, we don't really use the library
that we recommend here. So we should change the manual to reflect
real life.

https://govuk.campfirenow.com/room/412745/transcript/message/1169471680
Move breadcrumb related Sass into single file

This also fixes a layout issue on the search page.
Merge pull request #307 from alphagov/update-passphrase-entropy

Update passphrase entropy page and fix Sass issue