Add reporting on activity on technical mailing lists

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There was supposed to be a long comment here about how it doesn't seem like Hyperkitty supports searching by sender, but after trudging through a lot of API code (since nothing on https://lists.wikimedia.org/hyperkitty/api/ wrote anything about it), I've found the smoking gun: there's an undocumented API endpoint for it. So this is definitely possible and doable!

Placing this on Low priority for now while the more important services (GitHub, GitLab) get worked on.