WIP: Renames Tracking to EventTracking to match Ruby implementation.

What does this MR do?

As part of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31826, it was mentioned that we should use a more explicit name for our object. I was attempting to use the Tracking name, but have since adjusted things to consistently be EventTracking instead.

Documentation MR can be found at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32117 EE MR can be found at ---not opened yet---

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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