add database-backing for bookings #22
Description
Current situation: we use in-memory store for bookings
Desired situation: use a database so that booking server restarts don't lose booking information
Alternative: use a file backing and auto-export/reload script
When do we need it? The further in advance we allow booking, the more important this becomes. When we adopt LTI integration, we will have a motive to support bookings further into the future. Currently, bookings are lost if the user clears their cache, so we limit how far in advance bookings can be made so reduce the chance of a booking being lost.