Load parsoid content as XML

Parsoid returns valid XML, loading it as XML should have less potential
uncertainties than using loadHTML.

Change-Id: I63e8f062fe31e2455464e6d92436cd5ba54ad487
Workaround for relative links in /w/index.php

Relative links from parsoid, when rendered through /w/index.php,
don't currently work.  This is a temporary hack to include
a <base href="..."> tag in the page that allows these to work as
expected. This is not a long term fix because it will not work when
we show content from multiple pages in the same rendered output.

As this bug is not Flow specific, we should look into getting this
done more generically in core rather than hacking it into Flow.

Bug: 56988
Change-Id: Iefd9759595736bca64dd8361377dd638d968ac66
Merge "Workaround for relative links in /w/index.php"
Merge "Load parsoid content as XML"