Fix(LeftSidebar): Show hint when no filtered results

[DorraJaouad]

☑️ Resolves

Hint component was at the bottom of the LeftSidebar because the ConversationListVirtual is always present at full height.

• Added utility for conversation
• Refactored filter handling in leftSidebar
• Added captions for unread messages and unread mentions filter
• Add the empty content when there are no results

The filtering logic :
"Standard Condition" : if conversation has call , mentions / unread message and is currently active in the chat view.
"Special Condition" : if conversation is currently active, the only one in the list and has no flag.

• At the moment the filter is first applied:

  • Apply Standard Condition.
  • Check Special Condition:
    • If yes, hide the current conversation by returning an empty list. This handles the case of clicking the filter initially and showing empty content if there are 0 unread conversations
    • If no, show the filtered list

• After filters are applied (during subsequent filter navigation):

  • Apply Standard Condition.

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After	Rare but possible ( empty list)

🚧 Tasks

• Code review

🏁 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

[Antreesy]

Don't forget to sqash commit, your first one was completely overwritten

[Antreesy]

Left a last note (promise), how we can avoid going through the conversation list twice
Apart from that, looks and works nice!