* Add labs option to exclude unverified devices
Add a labs option which will, when set, switch into the "invisible crypto"
mode of refusing to send keys to, or decrypt messages from, devices that have
not been signed by their owner.
* DecryptionFailureBody: better error messages
Improve the error messages shown for messages from insecure devices.
* playwright: factor out `createSecondBotDevice` utility
* Playwright test for messages from insecure devices
* fixup! DecryptionFailureBody: better error messages
Use compound colour tokens, and add a background colour.
* fixup! DecryptionFailureBody: better error messages
Use compound spacing tokens
