Fix Email Reciever Field in Notification Hook

[J12934]

Description

The Email Notifier of the Notification Hook currently behaves differently as described in the Docs.
The docs indicate that channel.endPoint is supposed to be email recipient for the notification but actually has to be the name of a env var which then contains the email address.
This is a feature was added for the Slack & Microsoft Teams Notifier as these use webhook urls containing sensitive tokens.

I've updated the implementation to behave like the documentation.

Checklist

• Test your changes as thoroughly as possible before you commit them. Preferably, automate your test by unit/integration tests.
• Make sure npm test runs for the whole project.
• Make codeclimate checks happy

[malexmave]

Thanks for fixing this. I have a small comment on a typo. Also, since you mentioned the documentation: It is currently a bit inconsistent. At the top, it says "The general configuration of a notification looks something like this" and then shows an example that loads the env from a secret. However, for the email hook, it then loads the env directly from the file, with the username and password for the email account visible. Maybe we can take this opportunity to also fix that small part of the docs? (Reworking the rest of the docs for the hook is tracked in https://github.com/secureCodeBox/documentation/issues/182 and does not need to be addressed in this PR.)