WIP: Add ability to upgrade the subscription from the 'Group Billing' page

What does this MR do?

Ability to upgade sub from 'Group Billing page'

• Includes an 'Upgrade' button next to the 'Manage' button which redirects the user to the Customers Portal Upgrade/Checkout page;
• The 'Manage' button continues to direct the user to the 'Manage Purchases' page on the customer portal;
• The 'Pricing page' link takes the user to the marketing pricing page (https://about.gitlab.com/pricing/).

Closes https://gitlab.com/gitlab-org/customers-gitlab-com/issues/517

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 #517 (closed)