Make line breaks in <blockquote> behave like <div> (bug 6200).

This is an old, old bug: the earliest filed dup is bug 1857, on 2005-04-10.
See bug 51086 for a modern discussion, and bug 52763 for some non-obvious
consequences: indented text inside a blockquote must not trigger
creation of a <pre> block (unlike <div>).

This patch should bring the PHP parser and Parsoid closer together.

This also fixes (or works around) bug 15491, which is really a bug in tidy.
But because <blockquote> content is typically wrapped with <p> tags now,
we don't trigger the tidy bug (see
https://bugzilla.wikimedia.org/show_bug.cgi?id=15491#c7 for details).

Credit to Aryeh Gregor (https://bugzilla.wikimedia.org/show_bug.cgi?id=6200#c8)
and Vitaliy Filippov (https://bugzilla.wikimedia.org/show_bug.cgi?id=6200#c37)
for almost-correct patches for this bug, which saved me a bunch of effort.
Thanks to Subramanya Sastry for pointing out bug 52763 and preventing a
bunch of broken articles on enwiki.

Bug: 6200
Bug: 15491
Bug: 52763
Change-Id: I3696d4ab7b8ad6ebccf8483d6da1722353c1697d
Merge "Make line breaks in <blockquote> behave like <div> (bug 6200)."