Change current blockquote style to pull-quote class, and add proper blockquote

A change we made in H2 (and which I'm [copying for the Engineering Handbook](https://github.com/humanmade/red-book/pull/7#discussion_r182642731)) is to use blockquotes with consistent font-sizes, using indentation and borders to indicate their quote nature instead.

The current blockquote style in the pattern library is more useful for pull quotes instead of as a general-purpose blockquote style. We should update the pattern library to use this alternative style, and add a `.pull-quote` class (or similar) for the existing style.