Fix duplicate wikitext parse for displaytitle

Instead of doing a whole new parse for displaytitle, just get it from
the same API action=parse internal request that is used for returning
the content HTML.

Also escape entities as in OutputPage::setPageTitle(). The format of
OutputPage::setPageTitle()/getPageTitle() is a bit confused -- it's
mostly treated as HTML, but Article::view() sets it to the unescaped
title text -- a historical error which was unfortunately wallpapered
over.

Change-Id: I387c1ccd9a75f9c6c6a006d83235c6c1c3d1ecb5
Merge "Fix duplicate wikitext parse for displaytitle"