[IMP] [ADD] event: mail schedulign system.

This commit adds a way to configure scheduled emails on events. It replaces
existing automatic emails at confirmation and subscription. It proposes to
configure some schedulers, based on subscription or event, using a template
and a configurable timedelta.

A cron is added that regularly checks for event mailing schedulers to execute
and launch them. The granularity of send emails is therefore limited to the
granularity of the cron, aka 1 hour.

[TEST] Added tests for this feature.

Also cleaned a bit event, country_id is now a pure related instead
of a computed field trying to be a related. Also removed default templates
on event category. Configuration in the event itself should be easy
enough for most use cases.