More enhanced Mail sending to participants

[The-Ludwig]

currently, the mail sending to participants of an event is very basic. We should implement

• Preview and Test emails to admin
• Send one email to each participants, not all in BCC
• Use name of the participant in the mail
• HTML email, not only plaintext

[maxnoe]

Send one email to each participants, not all in BCC

The big advantage of this is that it is very simple: it's one email with a lot of recipients. With something else, we might run into rate limits of our mail servers pretty quickly and in general will need something like an asynchronous background worker sending out emails, since otherwise the UI will block until all emails are send out and it might die in the midst of sending out emails with no option of only sending to the failed recipients.

[The-Ludwig]

The big advantage of this is that it is very simple: it's one email with a lot of recipients. With something else, we might run into rate limits of our mail servers pretty quickly and in general will need something like an asynchronous background worker sending out emails, since otherwise the UI will block until all emails are send out and it might die in the midst of sending out emails with no option of only sending to the failed recipient

https://docs.djangoproject.com/en/4.2/topics/email/#send-mass-mail
I think this more of a feature we want to have in the django rewrite, not necessarily now. Django facilitates all of the asynchronous stuff and pitfalls for us, as far as I understand...