Ben/create availabilities

This functions. When you visit /spaces/:space_id/availabilities/new, you see a blank calendar. All the time ranges you select on the calendar will be written to the database as Availabilities for that Space.

_availability_calendar is a partial, so we probably will not need a 'New Availabilities' view or the ugly nested routing. But in the meantime this is the easiest way to implement the functionality.