Add a Upgrade button to group's billings page

What does this MR do?

Adds a "Upgrade" button to the group billings page which redirects to the upgrade page.

upgrade-workflow-from-group

TODO:

• Cleanup / add frontend tests
• Cleanup / add backend tests

Related to https://gitlab.com/gitlab-org/customers-gitlab-com/issues/518.

Required by https://gitlab.com/gitlab-org/customers-gitlab-com/merge_requests/392

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