[Cashtab] [Chronik Websocket Notifications] [Part 11] Ignore tx confirmed messages

Summary:
T2447

Depends on D11541

Ignore websocket messages that are not of type 'AddedToMempool'. Otherwise, notifications will also fire when transactions are confirmed.

These messages will be handled by different features in a later diff.

Test Plan:
npm start
Send yourself a tx and see notification
Wait for the tx to confirm (can monitor ecash.coin.dance or add a console.log(msg) statement to this diff ws config)
Note that you see no second notification

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D11554