Function format_display_name 

pub async fn format_display_name(
    db: &Database,
    user_id: &str,
    server_id: Option<&str>,
) -> Result<String>

Formats a user’s name depending on their optional features and location. Factors in server display names and user display names before falling back to username#discriminator. Passing a server in which the user is not a member will result in an Err.