Add holidays to the concept of "A business day" for notifications and other automations

Allow a user to select recurring and one-off holidays so that these could be treated as weekends are treated now: non-business days.

This would allow automation to be used to do something like automatically adding a comment to a ticket such as "Today is a company holiday, so you may not receive a response until tomorrow, but please call (xxx) xxx-xxxx if it is an emergency.", etc.