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.
However, it was not too hard to fix.

See bug 51086 for a modern discussion.  With this patch the PHP parser
matches the Parsoid output.

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.

Bug: 6200
Bug: 15491
Change-Id: Iecab69460c6aac36acfe2d9440dc5d3590de8b57
Merge "Make line breaks in <blockquote> behave like <div> (bug 6200)."