KSieve/Vacation: Add start date/end date when out-of-office reply script should be active

When the Sieve server supports "date" extension, we will show a date edits that
allow users to specify date range in which the out-of-office reply script should be
active. This is achieved by generating additional condition in the Sieve script.

REVIEW: 120540
GUI: New input fields in Tools -> Edit "Out of Office" Replies... dialog