feat(chatExtras): show subscribed threads in LeftSidebar

[Antreesy]

☑️ Resolves

• Follow-up to feat(threads): Add an API to get your list of threads #15646

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After

🚧 Tasks

• Pagination of thread requests
  • Currently API request results are mixed with all known threads
  • Need to store set of ids to get offset from it
  • Left a FIXME, not critical for beta

🏁 Checklist

• 🌏 Tested with different browsers / clients:
  • Chromium (Chrome / Edge / Opera / Brave)
  • Firefox
  • Safari
  • Talk Desktop
  • Not risky to browser differences / client
• 🖌️ 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