convert backupToInbox method from Promise to Future

[tomholub]

Maybe we could have a separate PR later to covert this backupToInbox to a Future. I'll file it.

Originally posted by @tomholub in #505 (comment)

[ivan-ushakov]

I'm not sure this could be done without changing the whole invocation chain starting from view controllers because inside view controllers this is used with Promises and await methods.

[tomholub]

That's possible. The goal is to refactor Promises away, but ideally do it in some smaller chunks of work so that the PRs are smaller, whenever/if it can be done that way.

[ivan-ushakov]

OK, I will work on it on weekends because it requires more time than I have during the day.