Capacities and occupancies for service and program occurrences

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[htysc]

@Damon-D-Ma The occupancy code requires a waitlist with a queue that has three methods like these:

• push(registration) - Insert this service/program registration into the waitlist queue
• pop() - Remove the next element of the queue
• remove(registrationId) - Remove the element with the given ID from the queue (from wherever it is, even the middle of the queue)

So the serviceWaitlist will need to have a queue of serviceRegistrations instead of clients.

There are four TODOs relating to waitlists in backend/services/genericData/programRegistration.js (and four very similar ones in backend/services/genericData/serviceRegistration.js). Once these are done we'll be much closer to completing the occupancy functionality.

[htysc]

@LesterLyu The part relating to service occurrences is complete and partially tested.

[htysc]

Ready to be reviewed