Add recurring reservations.
Add blocked_period table, model and upgradestep.
Adds a new function to the reserveration model to receive the dates it
targets.
Adds ability to block a time-period within a resource.
Add possibility to unblock periods for a reservation.
Remove unneeded parameter from event.
Fix upgrade step, add cols from mixin.
Make allocations check for blocked periods.
Rename module,  blocked_periods to blocked_period to be consistent with reserved_slot.
Sort imports.
Use bulk delete.
Make unblock work with tokens or reservations.
Make sure blocks are removed when their reservation is removed.
Display blocked periods in calendar.
Improve test cleanup.
Fix comment.
Remove blocked periods when removing reservations from sessions.
Make sure blocked periods are removed when sessions are removed.
Add recurrence database definitions and migration.

As discussed in seantis/seantis.reservation#14.
Adds a new function to the reserveration model to receive the dates it
targets.