Add public criteria for message event access methods

get_last_response_event and get_last_outgoing_event are used in various
places to determine which events to link to, use in queries etc.
Restrict them to refer to the last publicly visible event of the
relevant type, and rename them to make that clear.
Only include public messages in who_can_followup_to
Exclude hidden responses when calculating old_unclassified

Make old_unclassified_params method consistent with
last_public_response_event and associated methods.