First cut at cookie support in the web interface

Things that seem to work:
   * guest access
   * logging in

Things that are broken:
   * guests don't get a sensible error message when they exceed their rights,
     just nothing happens.
   * the redirection from the login page goes to an HTML redirection
     page rather than what you expected
   * if your cookie has expired the results aren't helpful

Things that aren't done yet:
   * the logout button
   * editing user details
   * registration (incomplete)