Fix User-Agent:
* debian/patches/series, debian/patches/ubuntu-prevent-distributor-user-agent-part-reset.patch:
  enable update of user-agent parts on startup again
* debian/rules, debian/vendor.js.in: create vendor.js pref file from .in template with data from
  /etc/lsb-release; add preprocessing of vendor.js.in to cdbs 'common-install-arch' target