Allow configuration of time slot durations

[sallyhall]

Time slots are a fixed 30 minutes right now. Ideally, we would be able to configure that, either by:

1. Configuring one or more static time slot lengths
   or
2. Using a duration attribute on Availability records, so a resource can be available for different length bookings during different time periods. This would allow, for example, a doctor to have availability for 15-minute appointments on some days, and 30-minute appointments on other days.