chore(MessagesList): Refactor scrolling

[DorraJaouad]

Includes

• Restrict Placeholder to not to create scrolling for the chat when it is there
• Message Highlight is event-driven now instead of ref-oriented, saves some computations
• Adjust fallback visible message to include only actual visible messages (e.g: not reaction system messages)

Follow-up:

• Removed extra steps in scrolling and improved remaining.

🏁 Checklist

• 🌏 Tested with Chrome, Firefox and Safari or should not be risky to browser differences
• 🖥️ Tested with Desktop client or should not be risky for it
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required