Function place 

pub async fn place(
    kind: &str,
    id: &str,
    parent: Option<&str>,
    session: &Session,
) -> ExitCode

Put an entity inside a portfolio, or take it out of one.

Two requests: the entity is read first for its version, so a portfolio that somebody else moved in the meantime is refused by Tracker rather than overwritten. The read also means a mistyped id fails before anything is written.