Notebooks: I can send an alert to a Sensu endpoint

We'd like to add the Sensu endpoint to the Notebook Alert panel.

### Details
Url: Text field with example (https://api.pushbullet.com/v2/pushes)
Token: Text field

[List of all endpoint fields](https://docs.google.com/document/d/1Tuo_HJNUrbbJCQ92qBa01UxFtytUzofLHYVNtGLPsaM/edit)
[Sensu Docs](https://docs.influxdata.com/flux/v0.x/stdlib/contrib/sranka/sensu/endpoint/)
[Source](https://github.com/influxdata/flux/tree/master/stdlib/contrib/sranka/sensu)
[Current API for adding a new alerting endpoint
](https://docs.influxdata.com/influxdb/cloud/api/#operation/CreateNotificationEndpoint
)
### Design