Reader: implement related posts train tracks in full post refresh

[nbradbury]

From @gibrown here:

"Do the related posts here implement traintracks events and railcars per #4462? I couldn't find them in the code, but maybe they are elsewhere. Also, based on recent Calypso experience it would be good to have the traintracks render events only get fired when the related posts get shown to the user (so when the user has scrolled that far)."

[nbradbury]

Notes: There are two click events now, calypso_reader_related_post_from_same_site_clicked and calypso_reader_related_post_from_other_site_clicked. There are render events for each item rendered so that the railcar payload for each item can be sent separately (ie: there are two from the same site and two from other sites, so four render events).