Add support for user namespaces on preview free user cap alert

What does this MR do and why?

Adds an alert on personal project overview and member pages for the free user cap - see #360019

this preview alert is behind the preview_free_user_cap feature flag, so there is no changelog.

Related to #360019

Note Sidecar MR !86718 (merged) that will wait until this MR merges and leverage some items in here.

Screenshots or screen recordings

User Namespace alert

How to set up and validate locally

1. Simulate SaaS and restart GDK
2. Update Application Setting

   bin/rails c
   ApplicationSetting.first.update(check_namespace_plan: true)

3. Enable the preview feature flag

   Feature.enable(:preview_free_user_cap)

4. Disable free user cap feature flag(so the preview alert shows)

   Feature.disable(:free_user_cap)

5. Visit any personal project http://127.0.0.1:3000/root/test-admin-namespace-project
6. Add at least 6 members.
7. Refresh the project members page you are on or go to the project overview page to view the alert.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.