Removed X-CS dependency in HTML content rewriting

* onMobileFrontendBeforeDOM() will now always produce the same result,
regardless of the X-CS, to allow future removal of cache variance.
* All <a> tags with href containing host or with class='image' will now
point to the Zero special page with two query params:
    to=URL & from=CurrentTitle
* The Zero special page adds varying on X-CS and X-Subdomain
* If special page has valid 'to' & 'from' params, it will show
confirmation page in case X-CS-based configuration demands it, or
silently redirects.

Change-Id: I012c43d2f5b9a2cfb4132f682c3fd76e48650811
Merge "Removed X-CS dependency in HTML content rewriting"