Cross-signing setup: verify by emoji

[dbkr]

After #11214

Add support for verifying a new signin by emoji.

• Implement SSSS key sharing
• Add the 'verify by emoji' option to the verify-new-signin flow, using SSSS key sharing to request the default key from other devices once verification is completed.

[turt2live]

Has this actually been done?

[jryans]

Hmm, we've kind of "accidentally" done pieces of it. A few things are missing still:

• There is no key sharing between devices, have to enter SSSS instead
• The flow does not match designs

[turt2live]

@jryans would it be worth me adding this to the QR code estimate?

[turt2live]

answer: no