Collapse quoted text in the threaded view of HTML messages

[ChristophWurst]

Feature Request

Inside the thread you, you don't really need the previous email's text as you can simply expand it there.

The exception would be inline replies. Then we should show the original text.

Technically HTML messages are a challenge. We don't render them directly, but via an iframe. So the HTML has to be possibly rewritten to achive this. This could be a hard and error-prone task.

Summary

Collapse/hide quoted text in plain text emails.

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See #3639 for plain text emails.