Track multiple transactions in the backgground

[softsimon]

• The backend needs a new WebSocket API for multiple transactions tracked at the same time
• The frontend need a global multi transaction tracker functionality that will live in LocalStorage and across pages
• A way to "pin" transaction to the global transactions tracker with UI similar to the mock preview below

Later, we probably want to extend this to address tracking as well later, and also adding multiple "shadow arrows" to show all tracked transactions below the mempool blocks.

[mononaut]

I added the websocket subscription for this in #4840