Add more verification for namespace UserCap availability

What does this MR do and why?

Adds a method to ensure the SaaS User Cap feature is only available for:

• Gitlab.com environment
• groups that have the feature flag specifically enabled for them

Also changes some instances of the Feature.enabled?(:saas_user_caps) to use the newly added method

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How to set up and validate locally

1. Find or create a group
2. Enable feature flag for the group

   Feature.enable(:saas_user_caps, group)

3. Check method:

   group.user_cap_available?
   
   => false

4. Modify Gitlab.com? to return true
5. Re-check the method:

   group.user_cap_available?
   
   => true

6. Disable the feature flag

   Feature.disable(:saas_user_caps, group)

7. Re-check the method:

   group.user_cap_available?
   
   => false

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.