www: more descriptive pagination

www: more descriptive pagination

Being an easily confused person, I find "next" and "prev"
ambiguous as to whether messages on the next or previous page
will be newer or older than the current page.  Clarify that for
the threaded /$INBOX/ view and search results.

For search results sorted by relevance, we'll use "[>= $SCORE]"
or "[<= $SCORE]" to indicate to indicate directionality.

This also fixes $INBOX/new.html for unindexed v1 inboxes.