Filter the display of content via a hook

This new hook, FlowCheckHtmlContentXss, allows for a callback
to easily be added which allows or rejects the display of
stored html content.  When we find the next xss in parsoid this
will allow preventing that html for exploting the next viewer.

Change-Id: I6587113073993b12c12499f896bd27501697d486
Mingle: 401
Merge "Filter the display of content via a hook"