Fix live content in page preview

[MizukiTemma]

Short description

This PR fixes rendering of live content in the page preview

Proposed changes

• Fix the region slug of the mirrored pages

Side effects

• I hope none 🙈
• The page preview functionality is available both on the page tree (👁️ icon) and on the page form. Please check both are working fine and in the same way
• Please check also, that the styling is fine (see Fix styling of page preview #3396 )
• Please check also (sorry for such many "please"s), that the rendering is working fine after changing the embedded content before and after saving the form.

Resolved issues

Fixes: #3426

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Pull Request Review Guidelines

[JoeyStk]

Thank you very much! This fixes the issue for me!

During testing I noticed the following formatting of the page:

Element	Is it shown correctly?
Marking text as bold	Yes
Marking text as italic	Yes
Marking text as underlined	Yes
Marking text as crossed out	Yes
Marking text as lowered	Yes
Marking text as superscript	Yes
Marking text in a color	Yes
Marking text with a background color	Yes
Headline 2	No
Headline 3	No
Unordered list	Yes
Ordered list	No
Text aligned to the left	Yes
Text aligned to the right	Yes
Text aligned to the center	Yes
Text aligned fully formatted	Yes
Inserted image	Yes
Inserted link	Yes
Inserted contact	Yes
Inserted special character	Yes
Inserted icon	Yes

Besides the ordered list and the headlines every seems to be formatted correctly! :)

[JoeyStk]

Since the mentioned issue will be fixed in #3433, I'll approve this PR 🎉