Allow setting moderation for a held message's sender.

This allows moderating a held message and setting it's moderation action at the same time.

It is a common practice to remove the moderation bit for senders on their first post to a mailing list, this should make it relatively much easier to do that.

WIP because it needs tests.

This is for you @msapiro ^_^

Fixes #127 (closed) Partly also relates to #339