S/MIME: Add proper warning when a signature is unverified

[st3iny]

Is your feature request related to a problem? Please describe.

A unverified S/MIME signature should be presented more noticeably to a user.

Describe the solution you'd like

• A whole line like for a mail with images: "Warning: Signature unverified, the sender might be impersonating someone!" – in bold though to highlight it
• Red crossed out lock icon on the left side next to it
• A button right next to it "More info" which opens the popover Skipped because it would duplicate information. We already have the popover in the thread header.

Warning: Signature unverified, the sender might be impersonating someone!