move thread namespace from database-layer to api.serlo.org

[AndreasHuber]

https://github.com/serlo/database-layer/blob/main/server/src/thread.rs shall be made obsolete by putting the required SQL queries directly into api.serlo.org instead of calling the database-layer. Use https://github.com/serlo/api.serlo.org/blob/staging/packages/server/src/database.ts for it. In api.serlo.org, make it work without the currently used "Serlo model".