Search settings should allow specification of number of characters to return

Currently search.php has hardcoded the default number of characters to be returned as 300. 

ACP->Search settings has a number of configurable items,. As an improvement the default number of characters to be returned should a setting that a board admin could change to something appropriate for their board rather than a hardcoded value in code. 

Discussion in https://www.phpbb.com/community/viewtopic.php?f=556&t=2547851 suggests that if board admin can establish a default setting, then perhaps users can set their own default search settings in their User Control Panel.