BE should not return secrets to FE

Requirements for this:
* discuss with @ebb-tide what FE wants here

Original issue:
```
Notification Endpoints have fields that contain secrets.
These fields should not be returned in plain text.
Let's discuss some way the FE can know that these fields have been set.
```

First discuss with @ebb-tide what the FE needs from the Notification Endpoints. Then inspect the endpoint fields that contain secrets and determine how their set state should be represented. Done means secrets are not returned in plain text and the FE can tell whether those fields have been set.

Written by the indexing model from the issue text.