follow-up(SystemMessageGroup) - improve behaviour

[Antreesy]

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Follow-up / Regression fixes for #9777:

• Showing in LeftSidebar in conversation
• Fixing the Unread messages marker (may be hidden in the list)
• Disable/remove/rework sorting system messages in the group

Feel free to add items to the list

[DorraJaouad]

Just a thought if we are keeping the read marker logic :
What if we just keep the combined system message not collapsed as long as LastReadMessageId is in one of its messages ?

For LeftSidebar, either we do the same ( keep it not collapsed by default if it is the last message) or we can reflect this "nested" format ( collapsed message > combined message > MessagesSystemGroup) in the store so we can get the combined message and render it in Conversation.

[nickvergessen]

This is solved for now, right?

[DorraJaouad]

No, only sorting messages was solved.

[nickvergessen]

Left sidebar I think not grouping is okay.

The readmarker could be because of mixed up sorting. But then it would be live already on our server and others reported just again that it happens, so yeah might still have issues.