nmbug-status: Add inter-message padding

nmbug-status: Add inter-message padding

We already had the tbody with a blank row separating threads (which is
not colored); this commit adds a bit of spacing to separate messages
within a thread.  It will also add a bit of colored padding above the
first message and below the final message, but the main goal is to add
padding *between* two-row message blocks.

                                               <--- new padding
  thread-1, message-1, row-1  (class="message-first")
  thread-1, message-1, row-2  (class="message-last")
                                               <--- new padding
    spacer tbody with a blank row
                                               <--- new padding
  thread-2, message-1, row-1  (class="message-first")
  thread-2, message-1, row-2  (class="message-last")
                                               <--- new padding
                                               <--- new padding
  thread-2, message-2, row-1  (class="message-first")
  thread-2, message-2, row-2  (class="message-last")
                                               <--- new padding