[Mailman-Developers] Migration of header_filter_rules

[Aurelien Bompard]

> You're right that model classes should never be imported outside mailman/model
> although there are a few violations of this rule.  It's less hard-and-fast for
> tests since sometimes that's just the most convenient way to handle it.
> [...]

Great explanation, thanks.
I'll send a merge request right now because I won't be able to do the
interface dance this weekend, and you may want to check out the rest
of the code. I'll create the required sets and interfaces on Monday.

Have a fun NFL season start! May all teams win! ;-)

Aurélien