Handle firewall log filter regex input better bug #3689

If the user inputs an invalid regex in any of the filter fields, then a page full of "warning" messages appear in the GUI, about whatever is invalid. 
If for some reason the user wants to match a forward slash somewhere, then they have to realize to escape it, doing "\/" instead of just "/". Be nice to this special case, because the user does not necessarily know that "/" is being used as the delimiter in the preg_match call. Turn "/" into "\/" (when the "\" is not already put in by the user).
For other regex issues, suppress the warning output, using "@". When the user inputs some invalid garbage in a filter field, an empty filtered firewall log table will be displayed, rather than screens full of PHP warning output.