[User Feedback] Use Case: Message Verification-Only Scenario

[martgil]

There were some users who were only interested in message verification.

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Reference:

https://mail.google.com/mail/u/human@flowcrypt.com/#search/required/FMfcgzQbgcNCxtdVJJBlJfkxccnZmlbc
https://mail.google.com/mail/u/human@flowcrypt.com/#inbox/FMfcgzQbgcTxFjCzfxPSBnVPcwKSGwhM
https://mail.google.com/mail/u/human@flowcrypt.com/#inbox/FMfcgzQcpKdTCsZrBDTxPrMfjjmlKGKk