Resolve "Guide users towards editing capabilities in review mode of Web IDE"

What does this MR do?

Fades the uneditable area of the Web IDE diff editors.

Why was this MR needed?

#45933 (closed)

Screenshots (if relevant)

It works 🙂

In "Review" mode, both editors are read-only, so they are both greyed out.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • [ ] Has been reviewed by a Backend maintainer
  • [ ] Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #45933 (closed)