Reference used images by absolute path in gerrit's css

Thereby, the images are found even when GerritSite.css gets inlined
from gerrit's new LDAP login URLs as
  https://gerrit.wikimedia.org/r/login/q/status:open,n,z

Bug: 46700
Change-Id: Ib68e9f1bf4d4e9994247c151ccab2bf1c82863c5