Hack to escape angle brackets in data-parsoid attribute

Parsoid is sending us some unescaped HTML in the data-parsoid
attribute. When we try to rebuild ref nodes (inline aliens)
this confuses Firefox which tries to sanitise the HTML by converting
<ref/> to <ref></span>.

As a temporary fix we can manually escape <>'s inside the
data-parsoid attribute.

Also in this commit the new MWReference nodes have been moved
to experimental as they are incomplete.

Bug: 47417
Change-Id: Ib6a0cfb880e769f28b42c9fa63ddc1abc75c399d