Allow reminder timing to be configured

[orenyk]

Feature request:

... we use the reservations system alongside a google form that schedules pick up times. We use the reservations emailing feature to remind people to schedule a pickup time, but the email is sent out on the day of the reservation, which can lead to some last minute reservations.

I’m wondering if there’s a way to adjust the time that the email is sent out? It would help tremendously if the check-in reminder was sent even one day earlier.