[4.0] Mail template: html to plain fallback and vice versa

[Fedik]

Pull Request for Issue #31919 .

Summary of Changes

Mail template, html to plain fallback and vice versa.
Also I have added "scroll up" for "test email" button.

Testing Instructions

Please follow #31919

Additionally, after send "test email" the page should be scrolled up, no need to do it manually.
So not forget to run npm install.

Actual result BEFORE applying this Pull Request

The error "empty body"

Expected result AFTER applying this Pull Request

No error, email sent.

Documentation Changes Required

nope

[Fedik]

Please someone add RB tag, as it was in the original issue.

[ceford]

I have tested this item ✅ successfully on 2e3bf2f

The email was sent to xxx@yyy.zzz using SMTP. You should check that you've received the test email.

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[richard67]

I have tested this item ✅ successfully on 2e3bf2f

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[richard67]

RTC

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[wilsonge]

Thanks!