feat: add email error handler

[trowik]

Resolves #571

[anehx]

We need to make this handler "opt-in" since not everyone will want this. Please make sure this will only be added if a certain env var is given. Also, the new env vars need to be documented.

[anehx]

Awesome! Good job 💯

[open-dynaMIX]

@trowik We are integrating django-generic-api-permissions right now. This will lead to extension code that possibly wants to use the django send_mail functionality. For this reason I would prefer to set the mailing settings no matter what, but only add the email logging handler if the env var ENABLE_ADMIN_EMAIL_LOGGING is True. This would more correspond to the expected behaviour of this env var IMO and would still allow sending emails.

The situation right now could lead to following scenario: I want to send mails from an extension. I use a hacky way of setting the mailing settings in django, but this would implicitly enable email logging.

[anehx]

@open-dynaMIX Very good point, I didn't think of this when reviewing.. @trowik could you do that?

[trowik]

@open-dynaMIX you're absolutely right.
I'm on it @anehx