Introduce a Vue-based UI for Pages

What does this MR do and why?

This MR introduces a Vue.js frontend boilerplate for editing the Pages settings for projects.

This will be needed soon as we add Multiple Version deployments to Pages, which makes the current view increasingly complex.

This MR just introduces the feature flag and boilerplate code to reduce the review workload for the actual UI change.

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Screenshots or screen recordings

No visible change for now

Closes #454320 (closed)