Add Seat Controls Block Seat Overages

What does this MR do and why?

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/501717, this MR introduces a new setting for Sign-up restrictions > Seat controls, namely Restricted access. With it, SM instances that have reached their licensed user count won't be able to add more Users (and generate overages).

This MR also removes the wip FF that was introduce as a placeholder for the actual settings.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

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

Screenshots or screen recordings

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

Test case	Screenshot (or recording)
https://gitlab.com/gitlab-org/gitlab/-/work_items/502411+
https://gitlab.com/gitlab-org/gitlab/-/work_items/503818+	user_cap_reached_user.mov
	user_cap_over_premium.mov
https://gitlab.com/gitlab-org/gitlab/-/work_items/503819+	restricted_access_premium_regular.mov
https://gitlab.com/gitlab-org/gitlab/-/work_items/511277+	restricted_access_premium_auditor.mov
https://gitlab.com/gitlab-org/gitlab/-/work_items/511278+	restricted_access_ultimate_regular.mov restricted_access_ultimate_auditor.mov
https://gitlab.com/gitlab-org/gitlab/-/work_items/511279+

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Set up gdk to run in non-SaaS mode (GITLAB_SIMULATE_SAAS set to false)
2. Set up a Subscription with a license (note: Premium and Ultimate will behave differently: see test cases above)
3. Visit admin/application_settings/general#js-signup-settings and select Restricted Access
4. Based on the scenarios, verify that new User can/not be added