Bootstrap's default only styles blockquote tags that also have the
class "blockquote". As Hugo's Markdown syntax for blockquotes only
produces bare blockquote tags, we should apply the same style to
those as well, as this is most likely what users expect.
When both sharing and Disqus blocks are present, the Disqus block
needs a top margin. Otherwise, if sharing is hidden, then the hr
following the blog post's article tag already provides enough of
a bottom margin.