Add success screen with further instruction after Sign & Export

[brusherru]

To help users go through sending MultiSig transactions experience we need to:

• close the verify tx popup (go back to "send tx" popup)
• open the special popup on top, which will say what to do next:
  • send the exported file to other parties,
  • or if you own other keys — import the file and sign again,
  • add details that signatures might be collected in sync or async way, so it is possible to combine all signatures as the last step by importing multiple files,