#12 link element for intracomment navigation

Mozilla now has a link toolbar builtin (altho it's
disabled by default currently). Slash has "home" and
"search" link elements, but "next", "prev", and "up"
would be very helpful while navigating comments.

Here's W3C's tutorial:
http://www.w3.org/TR/REC-html40/struct/links.html

[Rob Malda]

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I think Next & Previous should refer to Articles, and
not comments. But if someone submitted a patch that
did this, I suspect we'd accept it.

[Jamie McCarthy]

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We'd want to go to HTML 4.0 first...

[Chris Nandor]

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When looking at article.pl, we DO use Next/Previous to go to
the Next/Previous article. And Up goes to the section page.

We could add support for other pages: journals could use it
to go the next/previous journal (or journals, if viewing 10
at a time) for that user, and Up could go either to the main
journal page for that user, or to that user's info page, or
to the viewing user's main journal page.

For search, we could use next/previous to scroll through
results. Up goes back to main search page, for the options
you selected.

For arbitrary discussions, Up would go back to the thing
being discussed (article, journal, poll) and Next/Prev could
go to the next/prev comment. But what IS the next/prev
comment? Is it by CID, or by thread? Does it depend on
what mode you are using? And is there an efficient
algorithm that can be used to find the next/prev?

We don't need HTML 4.0, we have support for it now,
regardless. Some of these would be relatively easy (search
and journals), some not so easy; the main challenge for all
but comments is to get the data before calling header(),
which means in some cases some major code reorganization.