Function set_names 

pub async fn set_names(
    pool: &PgPool,
    actor: &Actor,
    display_name: &str,
    subtitle: &str,
) -> AppResult<()>

Sets the header name and subtitle (administrators). An empty display name restores the default name; a subtitle equal to DEFAULT_SUBTITLE restores the default subtitle; an empty subtitle shows none.

Invariants: admin only; the rows and the audit entry (branding.update) are written in one transaction.