Move labels.scss to page_bundles behind FF

What does this MR do and why?

This MR closes Convert stylesheets/pages/labels.scss into page... (#239844 - closed). In addition to that , this MR introduces these changes behind a new FF named page_specific_styles. Adding these changes behind an FF gives us room to fix them in a timely manner without the site looking broken ( since this is only UI ) and not cause any incidents 🤞

We can turn this FF on for some users and test it out for any uncaught regressions.

History for incident that led to the FF being introduced is here - 2023-09-11: Some project sites looks are broken (gitlab-com/gl-infra/production#16345 - closed)

We can remove the FF completely when all the files in _page_specfic_files.scss are fully migrated

Thanks to @leipert for the idea and the pairing session here 🙇

Screenshots or screen recordings

The following screenshots in both the tables are in the same order as mentioned below:

1. Admin labels : Admin > Labels
2. Group labels : Group > Manage > Labels
3. Project labels : Project > Manage > Labels

Before	After

Dark Mode before	Dark Mode after

How to set up and validate locally

1. Turn the FF on

   Feature.enable(:page_specific_styles)

2. Try visiting/playing with labels list at the project/group and admin level as mentioned in the screenshots
3. Check that no styling is broken
4. Check Creating new labels as well as well

• http://127.0.0.1:3000/admin/labels/new -> new admin label
• http://127.0.0.1:3000/flightjs/Flight/-/labels/new -> new project label
• http://127.0.0.1:3000/groups/jashkenas/-/labels/new -> new Group label

1. Repeat the steps for dark mode

Make sure nothing is broken.

Also , check that nothing is broken when the FF is off 🙂

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.