Scroll in Mempool Screen

[aszepieniec]

See #779. Let's make the mempool table scrollable as well. Motivation: in the future we might want more fine-grained control over individual transactions in the mempool with commands like "upgrade now" or "bump in priority" or "(re-)broadcast".

• make the "Mempool" screen scrollable

[aszepieniec]

Hint: implement ScrollableTable in the "Mempool" screen, in neptune-dashboard/src/mempool_screen.rs. Pattern match against the implementations of ScrollableTable in other screens such as "Peers".