Add logic for respecting browser DNT setting

What does this MR do?

It adjusts the tracking logic to respect the browser Do Not Track setting so it can be utilized in enabling or disabling of certain behaviors based on what a user has configured it to. The trackable static method allows asking purely this question, and the enabled static method utilizes it to determine if tracking is enabled at both an admin level, and user level.

Documentation updates are over in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32117

Does this MR meet the acceptance criteria?

Conformity

• [-] 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

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