Enhance: support e-mail moderation

The existing e-mail moderation method is rather inefficient, as all moderators get every mail that needs moderation.

It's not obvious whether a particular mail has already been dealt with by another moderator, unless they happen to have seen the mail come through. This means some duplication of effort.

As well as providing Accept/Allow/Reject functions, there could be buttons to add the sender to the appropriate 'deny' list and potentially mark the message as spam.

The issue names no files, tests, or entry points. Start by locating the existing e-mail moderation method and its Accept/Allow/Reject flow, then determine how moderator assignment, deny-list actions, and spam marking should work; done means moderators avoid duplicate handling and can use the requested actions.

Written by the indexing model from the issue text.