What problem would you like to solve? Please describe:
Adding a new notification provider currently requires MANY touch-points in code and a lot of overhead: DB migrations, new types in several packages, updating various switch statements, etc...

Describe the solution you'd like:
Make destinations generic so that things like listing Slack channels, sending messages to them, SMS, voice, etc... can be done through a separate service that can also be built/packaged with the core app.

Describe alternatives you've considered:
As it stands, the only way to extend providers is to update a lot of spider-webby code.

Additional context:
This will provide a clean way to add new providers quickly: #2290, #2102, #2049, #871, #552