Related Posts: Option to leave tracking URLs

Related Posts adds tracking args to the URL when clicking on a RP:

```
var args = 'relatedposts_hit=1';
args += '&relatedposts_origin=' + $( anchor ).data( 'origin' );
args += '&relatedposts_position=' + $( anchor ).data( 'position' );
```

But then we clean them up. It may be beneficial to some users to leave them so Google and other stats programs have easier access to use that for performance tracking.

Suggestion: Add a filter that sets a variable that gets localized into JS to determine if `cleanupTrackedUrl` is fired or not.

@xyu thoughts?

Start by locating the Related Posts JavaScript entry point and the cleanupTrackedUrl function shown in the issue, then trace how its tracking setting is localized from PHP. Done means a filter controls whether tracked URL parameters are removed, with coverage confirming both enabled and disabled behavior.

Written by the indexing model from the issue text.