Fix loading screen background and placeholder in immersive editor

What does this MR do and why?

This MR fixes two UI issues with the rich text editor in immersive mode:

• the loading state now has the same background as the editor itself (avoiding a little blink on load)
• the placeholder is now centered the same as the text

References

Closes #590813 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable wiki immersive editor with echo "Feature.enable(:wiki_immersive_editor)" | rails c
2. Open or create a wiki inside a project or group at "Plan > Wiki"
3. create a new wiki page. You may need to select "Switch to rich text editing"
4. Observe the loading screen and placeholder

MR acceptance checklist

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