refactor: Send emails from `EmailTransport` instances instead of class

[sveneberth]

This allows to store the credentials and other settings on the instance itself and reduce the config size. Furthermore I think this is more pythonic, combines what belongs together and makes abc meaningful.

This a small breaking change. Instead of

conf.email.mailjet_api_key = secret.get("api-mailjet-key")
conf.email.mailjet_api_secret = secret.get("api-mailjet-secret")
conf.email.transport_class = EmailTransportMailjet

you write now

conf.email.transport_class = EmailTransportMailjet(
    api_key=secret.get("api-mailjet-key"),
    secret_key=secret.get("api-mailjet-secret"),
)

---

Furthermore

• Enforce pep8 compliant snake_case names as requested by @phorward
• email.sendEMail is now email.send_email
• email.sendEMailToAdmins is now email.send_email_to_admins
• We use the deprecated package, which is not working smooth as expected, but this can be done later ...
• Add and update a bunch of documentation
• Fixing not used max_retries variable
• Introduce some constants (so things can be changed with monkey patching or are clearly reserved and fixed names)

Resolves #813

[sveneberth]

This pull request is in conflict with #1249, can you please integrate it here and fix the config first

Yeah, I expected this.

I will resolve the conflicts and do the renamings later.

[sveneberth]

@phorward @ArneGudermann @KadirBalku I'm now done, you can review now :)