Refactor & improve MultiSites plugin

[sgiehl]

Description:

• Reformats codes for better readability
• Disabled auto sanitize for API
• Replace als usage of Common::getRequestVar with Request class
• Adds more strict typ hinting / return types
• Adds tooltips to table header (refs Info tooltip for every table column explaining computations behind measures #17230)

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]

Nice clean up 🧹

A bit cleaner now, still lot's of responsibilities within the API class, but 100% an improvement.