#3448 Ignore signature for pubkey in protonmail

[rrrooommmaaa]

The fixed code doesn't process the signature attachment of a pubkey supplied by Protonmail
and hides it

close#3448

Also mocks text/plain and attachment rendering for GmailElementReplacer mock tests #4810

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Tests(delete all except exactly one):

• Tests added

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To be filled by reviewers

I have reviewed that this PR... (tick whichever items you personally focused on during this review):

• addresses the issue it closes (if any)
• code is readable and understandable
• is accompanied with tests, or tests are not needed
• is free of vulnerabilities
• is documented clearly and usefully, or doesn't need documentation

[rrrooommmaaa]

In extension's own inbox there is a duplicate block glitch

Let me create a separate test for it

[sosnovsky]

Great 👍