`admin/site` should be part of CS, not XP

[rymsha]

wile working on #9343 and #9314 we realized that it "special rendering" /admin/site does not naturally fit into XP. Content Studio still has to intercept rendering and all changes we make in /admin/site rendering ONLY needed for CS.

This complicates development

• CS has to wait for new versions of XP
• Changes in XP must be backwards compatible (support old and new versions of CS)

It also make introduction of new features (like nextjs rendering) harder: changes must be made in XP, CS and xp-nextjs codebase.

[rymsha]

This would give us a chance to remove 200 response in EDIT mode when controller is not configured. CS would know it by different means.

[alansemenov]

An issue to fix this in Content Studio: https://github.com/enonic/app-contentstudio/issues/5386