Message moderation should not apply to members of long standing

[mvl22]

We've just had a case of a message going into moderation (for some reason) posted by a member of long standing and with lots of previous genuine messages.

It would be sensible to disable automated message moderation if the member has posted say 3 messages (without cancellation) more than say a month ago.

This combination of rules should avoid the scenario where a new member posts a spam burst and that is let through. By contrast, member who has been on the site for more than a month and posted before that, with a set of messages that haven't been censored, is clearly genuine.