Function do_chat_assignment

async fn do_chat_assignment(
    context: &Context,
    chat_assignment: ChatAssignment,
    from_id: ContactId,
    to_ids: &[Option<ContactId>],
    past_ids: &[Option<ContactId>],
    to_id: ContactId,
    allow_creation: bool,
    mime_parser: &mut MimeMessage,
    is_partial_download: Option<u32>,
    verified_encryption: &VerifiedEncryption,
    parent_message: Option<Message>,
) -> Result<(ChatId, Blocked)>

Assigns the message to a chat.

Creates a new chat if necessary.