[ticket/7778] BBCode single limit

There are currently two hard limits for the number of BBCodes
allowed. One is enforced by the type of the `bbcode_id` column,
the other by an hard limit in `acp/acp_bbcode.php`. However this
limit can never be reached due to the size of the database column.

Suggested fix involves adding a new constant to define the max.
number of BBCodes (as with smilies) and chaning the database
column from a tinyint to a smallint to actually allow 1511 BBCodes

PHPBB3-7778

phpBB/includes/constants.php

@@ -173,6 +173,9 @@ define('BBCODE_UID_LEN', 8);
 // Number of core BBCodes
 define('NUM_CORE_BBCODES', 12);
 
+// BBCode hard limit
+define('BBCODE_LIMIT', 1511);
+
 // Smiley hard limit
 define('SMILEY_LIMIT', 1000);