REST client for driving Mailman 3

Comment 1 for bug 821438

Actually, these are going to be interesting, because individual IMember preferences are stored in multiple places, and there's a _lookup() method that returns the first non-None value. This would probably be fine for read-only access, but if you want to write changes, where do you write them? I think that if you POST or PATCH any of the attributes, it changes them on the member directly (i.e. overriding any other value).