WebAccess: IP-based rules and offline user fix

* Correctly handles a corner case of
  access_control_firerole.acc_firerole_check_user() where, when
  checking a guest user created with collect_user_info(None) (hence
  having the 'remote_ip' key set to '') against an IP-based FireRole
  rule, would have raised an unhandled exception. (closes #1233)

* Introduces a new register_exception() statement in order to early
  discover troubles with acc_firerole_check_user().
BibIndex: better wording for stemming in admin UI

* Improves the wording of the configuration form for the stemming
  setting, in order to better clarify how the system is affected
  by changing the stemming setting of an index.

Reviewed-by: Tibor Simko <tibor.simko@cern.ch>