[] The netspeed cookie should be enabled for 5% of users
[] Update onThumbnailBeforeProduceHTML to set src to a thumbnail that is 1/3 of the requested original thumbnail size (as represented by the image in src).
[] With JavaScript disabled the reader will only see the low-res image. The image may be pixelated but it can still
convey information. Suspend judgement on whether this is ethical until experiment has run its course.
[] With JavaScript enabled when an image is almost scrolled into view (at least 2 full viewport heights away):
* if srcset is supported we will turn data-srcset into a srcset attribute.
* if no srcset support we will replace the low res src attribute with data-src-lowres
[] The code for optimising images should only be loaded when the netspeed cookie is set.
[] Measure the difference between NetSpeed A vs NetSpeed B [TODO: determine how]