ispconfig3_wblist: don't hardcode wblist limit in config

[ghost]

If I understand this right, at the moment the number of entries for wblist for an email account is limited by the value set in config.inc.php - set to 15 by default.
However, would it not be better to get the configured limits for the client the account belongs to, check that the limit has not been reached yet, and only throw a "limit reached" error when adding another entry would get the client over the limit?