Add info to usage quota for free capped namespaces

What does this MR do and why?

For free namespaces where the free user cap applies, this

1. Adds a new card with a link to the billing page
2. Shows the maximum number of free seats available based on the cap

• Related Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/352638
• No changelog because it's behind a feature flag free_user_cap

Screenshots or screen recordings

Description	Screenshot
With subscription
Free plan without feature flag enabled (current behaviour)
Free plan with feature flag enabled (new/future behaviour)

How to set up and validate locally

1. Start your GDK

   export GITLAB_SIMULATE_SAAS=1
   gdk start

2. You want to have three different groups
   1. A free group
   2. A free group where the Feature flag is enabled

      bin/rails c
      group = Group.find(1) # Use the ID of the group you want to test
      Feature.enable(:free_user_cap, group)

   3. A group on a plan (go to http://localhost:3000/admin/groups, choose a group and set a plan). To be able to do this, you need to enable namespace plans:

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

3. For each group, go to the Usage Quota Page: http://localhost:3000/groups/GROUP_NAME/-/usage_quotas#seats-quota-tab

Note: As you have likely not set a correct subscription for the group with the plan, the "max seats for subscription" or "seats owed` number will be "0"

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.