Support enable pages for specific projects

What does this MR do and why?

Relate to https://jihulab.com/gitlab-cn/gitlab/-/issues/1222.
This MR is JIHU only and saas only.
Some private group requires Pages service,
We add a feature_flag so that managers can enable pages in specific projects

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. enable pages locally, you should see pages in sidebar
2. simulate saas
3. then pages should hide from sidebar
4. Enable jh_pages for a single project

 ::Feature.enable(:jh_pages, project)

5. the page should show only in this project

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.