Use the Mailman REST api to push mailinglist changes

One-sentence description

Use the Mailman REST api to push mailinglist changes

Why?

A new mailinglist sync will make for a more robust implementation

Current implementation

The current syncmailinglist.py is a hacky script which always pulls all the mailing lists from the website. It runs on an interval so it does not always reflect the latest mailinglists.

Desired implementation

Concrexit knows when a mailing list changes and can thus automatically update Mailman via REST api (which has official python bindings). With Celery this can be done in a seperate task to catch any errors. The new REST api is available in Mailman 3, and as we're currently running Mailman 2 I also created a related issue in https://gitlab.science.ru.nl/thalia/serverconfig/issues/204