feature: availability widget

[linhub15]

Select a room and a day to list all the available times, so prospective renters can see availability without needing to contact the hall manager and use up their time.

Rules

• each room can be associated to at most one google calendar

Queries

• room availability
  • check google calendar for availability (needs authToken + calendarId)
  • check system bookings for availability
  • return the time slots that are available

Mutations

• New google calendar event
• New system booking -> creates an event in google calendar (authToken)
• google calendar event deleted -> deletes associated booking if exists (HOW?)
• system booking deleted -> deletes associated google calendar event (authToken)