Verification: `other_user_id`, `other_device_id`, etc not exposed once verification complete

[richvdh]

After verification, element-web shows a completion dialog along the lines of "You've successfully verified <deviceName>: (<deviceId>)!".

The VerificationRequest class thows away details like the device ID once the verification completes. We could fudge around it in the js-sdk, but it seems like a useful thing to expose to other applications.