Add errbit notifications

Add `Errbit`/`Airbrake` notifications instead of using `ExceptionNotifier`.

- [x] Install `Airbrake` Gem and configure it
- [x] Use it alongside `ExceptionNotifier`
- [x] Configure deployments to use `Airbrake` as well
- [ ] Remove `ExceptionNotifier` config from deployments
- [ ] Uninstall `ExceptionNotifie` Gem from app

cc @alext