Feat(MessagesList): Soft update messages list AND add sticky date separator.

[DorraJaouad]

☑️ Resolves

• Fix #4610

• Refactor messages groups. It is now broken down by Date then by Author+time .

• Light refactoring for MessagesList, it was necessary 😶.

• Soft update messages list, keep the same object and modify only what was changed.

• Added sticky date separator.

[!NOTE]
As the date is sticky and the loading spinner needs to be in absolute position, it will be overlapped. Therefore, the loading spinner should be in another position other than middle, change the loading visualisation, or removed ( like in other messengers where the date separator is sticky)

Issues found to fix:

• [x] ~~Need more accurate calculation in the comparison between viewport and ul top values ( there is a slight positive gap that should be taken into account)~~
• [ ] The loading spinner ( see above )

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

https://github.com/nextcloud/spreed/assets/84044328/0c848091-eae4-4162-bb2b-f6188fa7fa22

🚧 Tasks

• [ ] Check Soft update thoroughly
• [ ] Reduce the opacity when it is shown sticky and/or make it darker in order to not to blend with system messages?

🏁 Checklist

• [ ] 🌏 Tested with Chrome, Firefox and Safari or should not be risky to browser differences
• [ ] 🖌️ 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