Add privacy setting allowing to randomise config ID on the backend

[michalkleiner]

Description:

When the option under Settings / Privacy/ Anonymize data is selected, every action and every visit has a random config ID assigned. It's always a new visit, the visitor is never known, and we don't have access to the last know visit (since there's none).

Ref. DEV-3639

Review

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• Documentation added if needed
• Existing documentation updated if needed

[caddoo]

Makes sense to me, I would want a second approval if possible though from someone who hasn't looked at it ( @mneudert )