MVC: Group and User Billing Page Improvement - Avatar and Name

What does this MR do?

• It shows the group's avatar in the group's billing page instead of showing the plan's avatar (free, bronze, etc.).
• It shows the user avatar in the user billing page instead of showing the plan's avatar (free, bronze, etc.), and it shows the user name below the avatar.
• It updates billing_plans_spec.rb to reflect the changes.

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #12797 (closed)

Closes #12843 (closed)