Add a class for better readability of list type within the code

Signed-off-by: Adam Warner <me@adamwarner.co.uk>

scripts/pi-hole/php/database.php

@@ -285,4 +285,9 @@ function remove_from_table($db, $table, $domains, $returnnum=false, $type=-1)
 	}
 }
 
-?>
+class ListType{
+	const whitelist = 0;
+	const blacklist = 1;
+	const regex_whitelist = 2;
+	const regex_blacklist = 3;
+}