Fix HIX value difference

[MizukiTemma]

Short description

This PR tries to keep the HIX value identical between pre save signal and tinymce.

Proposed changes

• replace \n with \r\n before checking HIX value.
• replace entity reference with numeric reference (example: ä -> ä) before checking HIX value.

Side effects

• Sadly I couldn't find a way to convert entity reference to numeric reference directly and needed some converting steps.
• It seems, so far observed in the test data, reference art of special letters does not cause difference in HIX value, but the style of new line does.
• If "\n" appears in the text by chance, for example when an user writes "Please use \n as line break" in the editor, simple replacement might replace it too, even though it is not intended as a line break. I testet it and "\n" tipped in the editor was not replaced, but I cannot garant it...

Resolved issues

Fixes: #2224

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[codeclimate]

Code Climate has analyzed commit 64eab46 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 28.5% (50% is the threshold).

This pull request will bring the total coverage in the repository to 76.2%.

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[timobrembeck]

Awesome, thanks a lot! 🙏

[JoeyStk]

Thank you for this important fix. I wasn't able to break it 👍