Check authentication token on gateway

Now the Apache on the gateway always requires a valid SSO session. This fixes
the bug with the 406 error after the SSO cookie expired, because the gateway
Apache now directly redirects the user to the login page.

It also simplifies the logout - before this patch the logout process was quite
complicated: first a redirect was executed to Gerrits logout page. Gerrit
removed its own session cookie and redirected the user finally to the cauth
logout URL, removing the SSO cookie.  Now only the SSO cookie is removed.

Change-Id: Iddab3882268baf0e849bdf98d856c13b478fd522