Create the first iteration of the “Contact sales” buttons experiment

What does this MR do?

Adds a “Contact sales” button to the paid SaaS plan options in the billing page for a top-level namespace. For this MVC iteration, the button will take the user to the About site sales page with a custom query param that will be used for tracking purposes. This is an experiment, so it will be behind a feature flag, and no changelog entries have been added.

Screenshots

User Profile Billing Page	Group Billing Page

On Bronze Plan	On Silver Plan	On Gold Plan

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• 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.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Addresses #219944 (closed)