- only remind weekly initially (not daily)
- root reminders on the forward port's creation, not the source's
merge date
- cap reminder interval at 4 weeks (instead of doubling every time)
- track reminders on forward ports, don't share between siblings
- remove `forwardport_updated_before` from the testing system, it's
now possible to just update `reminder_next` to a past date and see
if it gets triggered (it should to nothing on sources, or on forward
port in a state which does not warrant concern)
Fixes#801
