`courier#teardown` should cleanup all event listeners

[TuckerWhitehouse]

We can probably add a teardown method to the emitter, and just invoke that from courier (loop over all listener types and remove them) -- we could also try just setting listeners to an empty object... 🤔

https://github.com/Workgrid/workgrid-javascript/blob/master/packages/workgrid-courier/src/courier.ts#L92-L98

https://github.com/Workgrid/workgrid-javascript/blob/master/packages/workgrid-courier/src/courier.ts#L136-L140

https://github.com/Workgrid/workgrid-javascript/blob/master/packages/workgrid-courier/src/emitter.ts#L98