Werk #8093: New option for expiration of acknowledgements

Acknowledgements can now optionally expire. Multisite has been extended by an option to set a timespan when acknowledging problems. At the end of this timespan the acknowledgement is automatically removed and a log entry created in cmc.log. You can see an expiration in the details views about the comments of a host or service.

Expiration of acknowledgements mean something like "This should be repaired within time X". In combination with periodic notifications these acknowledgements will in effect stop the notifications for the configured while.

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