[client] Duplicates: put find fewer/more buttons on a new line and display...

That fixes "Duplicates: move the "find more / less" button to their own line" in #912 (closed) but I feel like "Duplicates: add visual feedback (short notification?) when the thresholds have been changed." would now be irrelevant: the visual impact of the button is way better than just updating a part of a long adjacent text (also a notification on each click would be annoying).