Superseded issues show a link to the superseder, but superseding
issues do not show a link to the issue(s) they supersede.

This would make the supersede mechanism unusable for rethreading
because the main application (importing old archives) is likely to
produce several partial threads with important content; explicit
cross-referrals should be easily possible even if the superseded
content doesn't appear in the superseder's view.

A selection menu of items (messages, files, etc) to export to the
superseding issue would be a good idea.