A11y: add functionality for prefers-reduced-motion to the timeline#876

Tanvez wants to merge 3 commits intoNUKnightLab:masterNUKnightLab/TimelineJS3:masterfrom

Harvard-ATG:a11y/prefers-reduce-motionHarvard-ATG/TimelineJS3:a11y/prefers-reduce-motionCopy head branch name to clipboard