Support private wikis by forwarding Cookie: headers to Parsoid

If configured to do so, the VE API will forward the Cookie: header to
Parsoid. This allows VisualEditor to be used on read-restricted wikis.

Bug: 44483
Change-Id: If4a0cf1e5785b332ec9b014b783412805cf8af75
Merge "Support private wikis by forwarding Cookie: headers to Parsoid"