'set up' in cross signing settings on unverified device prompts for passphrase three times

[dbkr]

• Have account with SSSS set up
• Log in on web
• Skip verification
• Go to settings -> security -> cross-signing, click 'Set up'
• Prompts for passphrase
• Enter passphrase
• Prompts for passphrase again
• Become confused
• Enter passphrase again
• Prompts for passphrase again
• Become angry
• Enter passphrase again
• Web is now verified & happy

[dbkr]

This could be fixed by calling accessSecretStorage from the CrossSigningPanel bootstrap method rather than calling bootstrap directly, but then should also restore the backup, ie. it should be running the exact same code as SetupEncryptionStore's usePassphrase, which then begs the question: in the simplest case where you skipped security when you logged in your new session, shouldn't it just bring up that same dialog again?

[dbkr]

To write this up in a bit more detail, what currently happens here is that when you press the 'set up' button in 'Cross signing' (when you have cross signing enabled but not active for the current session):

...you get this dialog:

...which basically just lets you get at your cross-signing by entering you passphrase rather than verifying. We could just display the same thing you get from the 'verify' toast: