Feature Request: Integrate Mailtrain as newsletter provider

[saschafoerster]

Mailtrain 2 is an openSource-newsletter-system, which would very well be a good part of the open Beabee ecosystem: https://github.com/Mailtrain-org/mailtrain
It has a very simple to use API (just the documentation is only visible with a working Mailtrain-server, as tokens are already filled in in examples): https://github.com/Mailtrain-org/mailtrain/wiki/Using-API
Partly it is visible in here: https://github.com/Mailtrain-org/mailtrain/blob/v2/client/src/account/API.js
I am not able to programm it, but I am ready to help with idea development and testing.