Move downstream pipeline rate limit to application setting

What does this MR do and why?

Adds new application setting for limiting downstream pipelines triggered by per project per user.

Adds UI to control and set the setting value

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

Setting on the /admin/application_settings/ci_cd page

How to set up and validate locally

1. Head to /admin/application_settings/ci_cd.
2. Check for the setting called Maximum downstream pipelines triggered per project per user.
3. Set the value you would like, default is 0.
4. Select Save.
5. Open rails console using rails c.
6. Check for ApplicationSetting.first.downstream_pipeline_trigger_limit_per_project_user_sha. It should have the value you just set.

Related to #425941