add to email filter blacklist from within a mail app

Currently, in the webmail login screen, before choosing a mail client, there is an "Email Filtering" option in which individual email account users can create filters one at a time. Filter content and criteria have to be manually entered. This is great for people who have one or two old enemies they'd rather not hear from, but it's not convenient or realistic at any meaningful scale.

A more realistic use case is that a user is getting spammed regularly from an address, perhaps dozens of different addresses, and wants to create filters with reference to specific senders. It should be possible to link to the blacklist from inside the mail app itself.

The details of this would need to be worked out, but it would be something like a "filter from this sender" button in each mail app that would grab the "from" field of each the checked messages (preferably multiple messages at once), and then route that information to the blacklist part of cpanel where it could be refined and edited on a rule-by-rule basis.

The blacklist UI desperately needs revisiting as well but I've marked that as a separate request.