Preview free user cap only for specific namespaces

What does this MR do and why?

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/365673

This adds the field include_for_free_user_cap_preview to namespace_settings to only show the preview of free user cap when

1. The Feature flag is enabled (preview_free_user_cap)
2. The namespace has include_for_free_user_cap_preview set to true

This enables us to rollout the preview per namespace, which would not be possible (performant enough) if we would use Feature Flag subjects.

Migration

Migrate

bin/rails db:migrate:main
main: == 20220712094945 AddIncludeForFreeUserCapPreviewToNamespaceSettings: migrating
main: -- add_column(:namespace_settings, :include_for_free_user_cap_preview, :boolean, {:null=>false, :default=>false})
main:    -> 0.0044s
main: == 20220712094945 AddIncludeForFreeUserCapPreviewToNamespaceSettings: migrated (0.0055s)

Rollback

bin/rails db:rollback:main
main: == 20220712094945 AddIncludeForFreeUserCapPreviewToNamespaceSettings: reverting
main: -- remove_column(:namespace_settings, :include_for_free_user_cap_preview)
main:    -> 0.0010s
main: == 20220712094945 AddIncludeForFreeUserCapPreviewToNamespaceSettings: reverted (0.0015s)

Screenshots or screen recordings

These screenshots are just here to know where the preview_free_user_cap has an impact on:

No cap, no Preview	Preview active	Cap enabled

How to set up and validate locally

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

1. export GITLAB_SIMULATE_SAAS=1
2. gdk start
3. Enable preview_free_user_cap as feature flag globally + use namespace-plans, and pick a group to test it on

   bin/rails c
   Feature.enable(:preview_free_user_cap)
   ApplicationSetting.first.update(check_namespace_plan: true)
   
   # find a group ID
   group = Group.find(GROUP_ID)
   group.namespace_settings.update_columns(include_for_free_user_cap_preview: true)

4. Go to the Usage Quota Page (Group > Settings > Usage Quota) http://localhost:3000/groups/GROUP_NAME/-/usage_quotas#seats-quota-tab and be able to see the toggles and the banner
5. If you want to check the free_user_cap

   bin/rails c
   Feature.disable(:preview_free_user_cap)
   Feature.enable(:free_user_cap)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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