Check ins emails should be temporarily disabled when on extended break

[Wimsy113]

• Vacation mode added to to accounts with check_ins enabled
• Should be able to toggle vacation mode on and off from account menu
• Vacation mode should accept date from_first_day_off to_last_day_off
• Should be able to extend or shorten vacation mode as see fit
• Vacation mode should also deactivate the missing_check_in_links on the dashboard for days not checked in

[zspencer]

I like this idea, but I think we may want to wait until people complain that they're getting emails or they start unsubscribing before we prioritize working on it.