Do less aggressive subject nesting

[Humbedooh]

This addresses #427 by only allowing sort-by-subject to
happen if we can establish that an email is a reply.

[sebbASF]

I don't think it makes sense to match by subject even if there is a reply prefix indicator.

If there are two identical subjects, they are likely to both get replies, so they will still get tangled up.

[Humbedooh]

That will only be true if there is no IRT or References header.
It's by no means perfect, but would you prefer the alternative that we could show 100 'threads' that are essentially the same thread?

[sebbASF]

There will only be 100 unconnected threads if the mailers are broken and don't provide Reference headers. Are there many such broken clients?

Anyway I think it's better to have broken threads than to merge unrelated threads.
In the latter case it's easy to lose sight of the misfiled mail (indeed that's what alerted me to the problem; I could not see the mail I had just sent).