Prevent landings during soft freeze

[jgraham]

If we land during a soft freeze it can end up creating annotations that aren't correct for beta and so cause additional work for the sheriffs. Therefore it would be good to pause landings during the soft freeze period. The calendar for soft freeze is available at https://wiki.mozilla.org/Release_Management/Calendar

One thing to consder is what happens if we start a landing before the soft freeze, but by the time it's done we're in a freeze? In this case we probably want to retry the landing later on, but also we want to redo the latest try push. Maybe try pushes for landings should become invalid after a couple of days?

[whimboo]

I assume that we wanna use the ICS file instead for parsing the start and end dates of the soft-freeze:

https://www.google.com/calendar/ical/mozilla.com_2d37383433353432352d3939%40resource.calendar.google.com/public/basic.ics