Sign-Up email is not sent to admin for registration pending approval

[congineer]

Bug description

Upon user sign-up no email to admin(s) is sent.

Expected behaviour

Admin(s) should get an email about user's Sign-Up attempt.

Any strategy to fix this:

• a hint on how to properly configure the notification email to admin(s)
• a hint on how to patch the NativeAuthenticator code, e.g. similar to the self-serve approval way #L343

Actual behaviour

Upon user sign-up a message #Line125

Your information has been sent to the admin.

is being displayed, but no email to admin is triggered to localhost Postfix email server.

How to reproduce

1. Just 'Sign-Up'
2. Observe the Your information has been sent to the admin.
3. Assume the admin actually got the email
4. Wait indefinitely till the admin checks your request in the JH Authorization area page

Your personal set up

• OS: Ubuntu
• Version(s): ubuntu 22.04

Full environment

``` # paste output of `pip freeze` or `conda list` here ```

Configuration

```python # jupyterhub_config.py ```

Logs

``` # paste relevant logs here, if any ```

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