fix(chat): Add a repair-step to handle the last-read-message=0 case

[nickvergessen]

☑️ Resolves

Repair step doc block

• In previous versions of Talk, the value 0 was used to mark a chat as
  completely unread. This meant that the next time a client/browser loaded the
  chat it would start all from the beginning. However, there were various
  issues over the time that made the frontend getting null or undefined
  into the wrong place and then accidentally loading 8 years of chat from the
  beginning. So it was agreed that the frontend manually blocks loading with
  last-read-message=0 and the special cases use -2 for this situation.

🛠️ API Checklist

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not possible
• 📘 API documentation in docs/ has been updated or is not required
• 🔖 Capability is added or not needed

[nickvergessen]

/backport to stable30

[Antreesy]

LGTM