Re-introduce read_admin_projects custom admin role permission

What does this MR do and why?

In !201406 (merged) read_admin_projects custom admin permission was removed because the current Vue implementation of /admin/projects doesn't support the expected behavior of the permission.

This MR re-introduces the same permission with an accompanying feature flag (custom_ability_read_admin_projects) to make it unavailable by default. The same feature flag will be used to de-risk the introduction of the frontend implementation to support the permission (see !201259 (merged)).

References

Allow read-only access to `Admin Area > Project... (#557844 - closed).

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.