Add Seats tab to conditionally render in CE

What does this MR do and why?

This implements Add Seats app to conditionally render in CE (#371822 - closed)

After we finish with #440195 (closed), i.e. after we merge !143633 (merged) and !143637 (merged), we're now able to migrate other tabs. The migration here is simpler because we just need to conditionally render this tab.

In other words: This tab will be hooked to an App that is in CE but will only render in EE.

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

FF off	After ( FF on )

How to set up and validate locally

We need to validate this in 4 scenarios:

1. GitLab EE Unlicensed: this is probably how your GDK is right now. If you have a license in your GDK, remove it in https://gdk.test:3443/admin/subscription
2. GitLab EE: by having a test license in your GDK.
3. GitLab SaaS: setup a license like the above and simulate SaaS
4. GitLab CE: by executing
   1. export FOSS_ONLY=1
   2. gdk restart
   3. Tanuki gdk website icon (the one in the browser tab) should be blue, then you're in CE

Now go to any Group Usage Quotas: https://gdk.test:3443/groups/flightjs/-/usage_quotas and check if your local setup matches the expected behavior.

• To enable the FF: Feature.enable(:usage_quotas_for_all_editions)
• To disable the FF: Feature.disable(:usage_quotas_for_all_editions)

Expected behavior:

GitLab Distribution	Expected behavior FF off	Expected behavior FF on
GitLab EE Unlicensed	Renders a 404 page	Renders the seats tab alongside the others
GitLab EE	Loads the page normally	Renders the seats tab alongside the others
GitLab SaaS	Page loads normally	Renders the seats tab alongside the others
GitLab CE	Renders a 404 page	Renders only the storage tab