Add warning for suspicious messages

[sosnovsky]

Suspicious messages has Received-SPF: softfail header, for such messages we need to show warning before message body:

Potentially suspicious message

It wasn't properly verified by the sender, so its authenticity can't be confirmed.
Be careful - avoid clicking links and downloading attachments, or sharing personal info.

ref: https://github.com/FlowCrypt/flowcrypt-security/issues/270
android: FlowCrypt/flowcrypt-android#2670

[ioanmo226]

@sosnovsky Current implementation looks like below(Gmail app style).
Let me know if you want our Android app style which has yellow left border instead of background.

[sosnovsky]

Current implementation looks great, let's use it 👍