model: remove BaseModel class The only remaining purpose of this class was to provide the "sa" field, allowing a custom SQLAlchemy session to be used for model operations. However, this never actually worked, nor was it used anywhere. There's always a global SQLAlchemy session associated with the current thread; using another session for a single function call does not make any sense (as sessions cannot be mixed), unless the code works carefully to ensure the two sessions (and all objects loaded from them) are kept completely separate. Suffice to say that Kallithea does no such thing, thus there's no need to pretend to support multiple concurrent sessions.