Use a boolean for the waitlist state

Problem/Motivation

The DX of the Registrant::getWaitlist() and Registrant::setWaitlist() methods is poor. The documentation mentions that an integer value should be passed, but in fact a string value "0" indicates that a registrant is not on the waitlist, and "1" indicates they are on the waitlist.

Proposed resolution

Let's use a boolean value.

API changes

This changes the types of these methods. We should throw deprecation warnings in 2.0.x, and enforce the boolean type in 3.0.x.