[Mailman-Developers] Migration of header_filter_rules

[Aurelien Bompard]

OK, I have something that follows these requirements (except the
site-defaults + site-config thing, currently site-wide checks take
precedence).
If you want to look at it :
https://gitlab.com/abompard/mailman/commits/import-header-matches

If you have some time Barry, I'd like your review on some things that
I'm not sure I'm doing right:
In the code and in the tests, I'm importing the new model object,
HeaderMatch. Is it OK? Should I go through the corresponding interface
instead? It looks like models are almost never imported from
throughout the code (except in the model module), and I don't know if
that's a best practice that should be followed here. If it is and you
have details on the reasoning, I'm interested :-)
Example : https://gitlab.com/abompard/mailman/commit/7a14a5f42af35185bee19b8e09c6e272fd85282e#4e170b02877ebb99f9ecbcd5fd2f6a50ee49827f_51_51

Thanks !
Aurélien