Open hub with promise from IPFS

[svub]

Nimiq Hub can take as parameter a promise, so, it's possible to push the order to IPFS and instead of waiting, calling the Hub directly with a Promise that resolves when IPFS returns the file hash. This way, Nimiq Checkout can start syncing while we wait for the hash from IPFS.

Besides being nice enhancement, if IPFS takes long, the browser will refuse to open the window (300ms limit for user interaction handlers!) - so this fix will avoid such an edge case by opening the window right away.