Task:
Highlight diffs I haven't seen since my last visit

Motivation:
Sighting an article, I am especially interested in those revisions that I haven't seen from the article yet, since those are the ones that might contain vandalism / inappropriate changes.

Background:
This is part of adding the revision slider to the revision diff page. The revision slider is a feature that adds a visualization of all revisions to the compare screen.
The revision slider was originally prototyped by the WMF Community Tech team and then handed over to the WMDE TCB team, as it is also a solution to fulfill a wish of the German Community Wishlist: https://de.wikipedia.org/wiki/Wikipedia:Umfragen/Technische_W%C3%BCnsche_2015/Artikel#Anzeige_aller_Bearbeitungskommentare_im_Diff