Fixed scheduled reminders (and improved them)

After a couple of months of this feature not working due to an external dependency, we have fixed it
. The solution leverages the Google Apps Scripts capability, which can run processes in the background.
The only drawback is that we can only set up hourly runs, which means that the scheduled reminder will run only every hour, somehow at a random time. Therefore we cannot ensure the exact minute the reminder will be sent
, only that it will be sent at around X hours before the event.
We also took the time to make the scheduled reminders configurable up to 72 hours before
 the event happens (as requested here).
We also added {{hangoutLink}} as a placeholder
 for the scheduled reminder template (as requested here).