Enhance: add a way to filter mails from view

Where a list includes a mix of automated and human communication, it can be difficult to follow the messages. It would be useful to be able to add a filter to exclude such items from view.

[Ideally such messages should be directed to a separate list, but that is not always possible, and cannot be done retrospectively]

If the user is logged in, it would be useful to be able to save such filters.

Note: it needs to be obvious when a filter has been applied.

The issue names no files, tests, or entry points. Start by locating the mail-list view and the existing login or saved-preference handling; done means users can exclude automated or human messages, see clearly when a filter is active, and logged-in users can save filters.

Written by the indexing model from the issue text.