Add the Upgrade button to the User Billings page

What does this MR do?

Simplifies upgradable plans workflow presenting an Upgrade button which redirects to the customers portal Upgrade page:

Screenshots

Current user + Free plan

Current user + Early Adopter plan

Current user + Bronze plan

Current user + Silver plan

Current user + Gold plan

Closes https://gitlab.com/gitlab-org/customers-gitlab-com/issues/518

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