www: improve navigation around contemporary threads
Sometimes it's useful to quickly get to threads and messages
which are contemporaries of the current thread/message being
focused on. This hopefully improves navigation by making:
a) the top line (where $INBOX_DIR/description) is shown
a link to the latest topics in search results and
per-thread/per-message views.
b) providing a link to contemporaries ("~YYYY-MM-DD") at
around the thread overview skeleton area for per-thread
and per-message views