Bug 44483: Forward Cookie header to API

By forwarding the Cookie header from requests to the API we get to load
content from private wikis. Also send the right headers to prevent Varnish
caching of private content.

TODO:
* Figure out a solution that works for job updates (no cookie there) so that
  private wikis can also be pre-cached.

Change-Id: Ia7f4bd9e8b39f12fd1e31cdf7b44643aa33739ab