Define interaction between use_last_activity_as_completion_time and if_completed_before (See also #358 on Github)

Return False for anonymous users if only if_has_participation_tags_any or if_has_participation_tags_all
is not empty.

add view_participant_masked_profile permission, which is not activated by default. Display "Me" for the request user when masked.
Enable masking username in email title and contents.