By default, hide referrers on public stats

[ozh]

By default we're showing referrers on public stat pages -- example : https://yourls.org/ozh+ see on "Traffic Sources"
To prevent any 2000's old school referrer spam, or maybe for privacy, we should display these only if the stat pages are private

So, if yourls_is_private() !== true, don't display the whole "Traffic Sources" tab. Make this filterable so that anyone absolutely willing to display if can do

[ozh]

Ping @AntoineFr for info