Improve verifications

[VAWVAW]

Improve verifications in the following ways:

• refactor state keeping
• don't auto-accept verification requests
  • don't force users to use iamb for verification
• start SAS verification flow
  • verifications between two iamb clients were broken because no one would actually start_sas()
• show more information about the flow state
  • requests we have sent, requests we can accept, requests waiting for the other user, requests that were completed by another device
• add verification via showing a QR code