Provide a better UX to signal that the deposit flow is asynchronous

[accordeiro]

In most real-world cases, the phase between finishing the interactive flow and completion might take from 10 minutes to 3 days. We need a better UX on the sep6-demo-client to reflect that.

A couple of ideas to solve this:

<@tomquisel> I think the best practice during this phase is for the wallet to display information about the deposit and say that it's processing.

<@accordeiro> After user finishes the popup flow, we display a screen on the wallet that shows the deposit info that was provided through the postMessage callback. Then we can provide a button on that screen stating “I’ve finished making the deposit”, which would trigger the call to the /GET /deposit/confirm_transaction/? This way, we can make it clear that this is an asynchronous process.