Track membership change events

[KOVALW]

The (currently unused) ReplaceWith itinerary modifier method does not assert that setting id's should be consistent across default and modified itineraries. This will pose an issue with changes in forecasted infectiousness both for individuals who are infectious and when new people increase the membership count of settings that have ongoing forecasts. We need to create a MembershipChangeEvent that is listened to efficiently by the infection propagation loop.

Tasks

• Create a membership change event that lists settings with increased or decreased membership and the person who moved
• Create a data container that tracks forecast PlanIds and cancels them when appropriate
• Make this method efficient by only listening to events that could alter infectiousness maximum rates (setting types with nonzero alpha, only the relevant setting ids, etc)