Extensibility

[fopina]

Allow custom notification systems to be added by apps.

This would require making the service field "soft choices" (drop constraints), so subscriptions can have invalid services (as they could be removed after subscription was configured), but just log normal notification error for those.

Take advantage of this new dynamic service choices to only show services that are configured (ie: slack is not an option if token is not in settings)

[fopina]

If services can be added externally, consider moving slack to its own package to cleanup required dependencies #9 (comment)