Make group mailinglist mode real mailinglist mode like in categories

For now only categories enable “real” mailinglist mode, wich means you really get ALL mails even your owns and if you answer a thread online. This doesn’t work for mailinglist mode in groups [1]. :sadpanda: It is hard to explain to our users why there is different behavior. Moreover a lot of them won’t switch from mailman because of the “getting all mails”-feature not implemented in discourse yet.

[1] Mailing list mode "Send me an email for every new post" doesn't send mails for your own posts - #3 by agit