[ticket/12807] Changes PRUNE_FREQ to PRUNE_SHADOW_FREQ/_DAYS

PHPBB3-12807
[ticket/12807] Adds missing language variables language/en/acp/forums.php

PHPBB3-12807
[ticket/12807] New variables for _EXPLAIN

PHPBB3-12807
Merge remote-tracking branch 'Crizzo/ticket/12807' into develop-ascraeus