Notification of completion of coinjoin events or wallet updates

[AdamISZ]

In jmclient.wallet_service.WalletService.transaction_monitor we allow clients of WalletService to register callbacks on transaction events.
If it were possible to somehow have the react client subscribe to events this would be a big step forward in having the UI kind of "work", since e.g. a coinjoin completing (unconfirmed) on the network could be shown in a dialog.

[AdamISZ]

7c79f5c is a partial solution, but certainly not a complete one: client receives updates of state via polling and displays on screen. (Notably: /display will also get a live version of the wallet contents whenever the page is reloaded as it calls the backend, which is updated itself).