Bootstrap: format <blockquote> without `class="blockquote"`

[xenoterracide]

downstream bug https://github.com/alanorth/hugo-theme-bootstrap4-blog/issues/50

using v4.alpha6

I think that like bootstrap 3, that just having

<blockquote>
   <p>foo</p>
</blockquote>

should be a valid blockquote, I can see wanting or needing the class for more advanced things, but semantically rendered html from sources like markdown should work without adding the class, imho.

[mdo]

We're sticking with the class name to limit scope of our styles.