[Merge] lp:~coolyashish/postorius/mass_unsubscribe into lp:postorius

[Terri]

Review: Disapprove

I've merged a similar patch that includes this functionality as well as an unsubscribe all button, so I'm going to reject this one.  Thank you so much for your contribution, though, it's still decent code and you should be proud of it!
-- 
https://code.launchpad.net/~coolyashish/postorius/mass_unsubscribe/+merge/252943
Your team Mailman Coders is subscribed to branch lp:postorius.