Add transaction status notifications

[satello]

Add the Pending Transaction and Transaction Complete notifications. Alternatively we can leave this up to the sellers website as they get a callback when a transaction is submitted.

[epiqueras]

They don't get a callback when a transaction is completed, though. That happens after the payment is made.

[satello]

I am talking about these

https://www.figma.com/file/b7DEUKDeHlOl4ocDTMv7UIhZ/Kleros?node-id=7790%3A43219