Don't count merged tickets for amount_check api

Hello!

We are using the amount_check api to keep an eye on ticket counts raising to unusual levels. If a malicious customer starts spam-creating tickets it is a good way to be notified about it early on before having a large amount of tickets to manually clean up.

It would be cool if the check could exclude already merged tickets, as usually merged tickets were mainly intended as a reply to an existing ticket or in the above spam-scenario merely duplicates.

Eg. something like this: