Fix quote marks on blockquotes

The correct format is a quote before every paragraph of a blockquote and
a single close quote on the last quote of a blockquote.

Added the close quote with a class due to older IE's not understanding
the :last-child selector.
Added missing include

The admin pages were broken when loading the styling: we were missing a mixin
Merge pull request #305 from alphagov/blockquote-quotes

Fix quote marks on blockquotes