Follows-up on earlier discussions about the relationships of
/availabilities and /schedules.
`GET /schedules/:id` returns a schedule with associated availabilities
in the `availabilities` property. It gives more context and less
consumer work to perform GET actions using this endpoint. Other
endpoints of this collection do make sense.
Proposing also to rename the /availabilities collection to
/availability; given after this it always involves one and only one
/availability record in CRUD.
