Scheduler should avoid bank holidays

[sihil]

It makes sense that the scheduler introduced in #476 should avoid deploying on bank holidays.

One approach for doing this might be to download https://www.gov.uk/bank-holidays/england-and-wales.ics at build time and add it to resources. This could then be parsed at startup (perhaps that should be verified at build time?) and quartz configured appropriately using a HolidayCalendar.