Add header referer field under unused signals report

Presently, there is no way to figure out from which site a particular signal is coming from via the Unused signals report.

For example: c_article_title=top news doesn't tell us which site triggered this.

So, if there are 5 sites, we need to setup 5 traits and then wait to see which one populates and then delete the rest which is a very inefficient option. Manual lookup is not feasible in case of publishers with multiple sites and while creating traits in bulk.