Custom hours for each SLA

Let each SLA policy use either business hours or calendar hours (24/7), so different customer contracts can be supported in the same workspace.

For example, a critical ticket received on Saturday at 20:00 should be due on Sunday at 20:00 for a customer with a 24-hour calendar-time SLA. For another customer with a four-business-hour SLA and Monday-Friday, 09:00-17:00 office hours, it should be due on Monday at 13:00.

Applying a different SLA through Workflows should also apply that policy's time calculation, without changing the office hours used by other customers.