Indicate what mailing list a visible email is shown from

When searching for an email, if its included in multiple email lists, the individual message view does not indicate what list its from. This makes linking to them for archival purposes, e.g. board emails, hard to follow and confirm.

Start at the individual message view reached after searching for an email, and trace how messages associated with multiple mailing lists are represented. Done means the visible message identifies which mailing list it came from, making archived links such as board emails easy to follow and confirm.

Written by the indexing model from the issue text.