Move `StopSignal` from `SuperSpreader` to `TrackBallast`

[benjaminoakes]

Background

Please see doximity/track_ballast#19

Proposed

• Add a deprecation warning for anyone who is using StopSignal because it will be removed in v1.0
• Change any references to StopSignal (e.g. in the documentation) to refer developers to TrackBallast. This may make sense to wait to merge until there’s a production-ready release of TrackBallast that includes StopSignal as added in Move StopSignal from SuperSpreader to TrackBallast track_ballast#19