Add manual renew button to billings page

What does this MR do?

It adds a renew button to the billings page -> subscriptions table, where it points the user to the renew page on CustomersDot.

The URL used for the renew button is defined in gitlab-org/customers-gitlab-com!2272.

The change is being handled by saas_manual_renew_button feature flag, rollout issue.

Related to gitlab-org/customers-gitlab-com#2255.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Separation of EE specific content

Availability and Testing

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