Eliminate transition effects from thumbnail loader

During A/B testing we found a significant preference (increased engagement
time, reduced downloads of original PDF for local viewing) for the version that
doesn't animate loading thumbnails.

Profiling shows that the transition effects hit at a pessimal time when the
compositor is busy rendering the main PDF page view, and rendering thumbnail
previews, causing the main scroll view to stutter. With the transition removed
scrolling is perceptibly smoother in Chrome and Firefox. We also hypothesize
that the transition effects added to a perception of slowness but presumably
the aforementioned stutter is the primary issue here.

web/viewer.css

@@ -1127,7 +1127,6 @@ html[dir='rtl'] .verticalToolbarSeparator {
 }
 
 .thumbnailImage {
-  transition-duration: 150ms;
   border: 1px solid transparent;
   box-shadow: 0 0 0 1px rgba(0, 0, 0, 0.5), 0 2px 8px rgba(0, 0, 0, 0.3);
   opacity: 0.8;
@@ -1139,7 +1138,6 @@ html[dir='rtl'] .verticalToolbarSeparator {
 .thumbnailSelectionRing {
   border-radius: 2px;
   padding: 7px;
-  transition-duration: 150ms;
 }
 
 a:focus > .thumbnail > .thumbnailSelectionRing > .thumbnailImage,