[A11Y] Use `prefers-reduced-motion` to calm animations

[davwheat]

People with inner ear conditions can become nauseous and physically sick from web animations.

We should NOT remove animations, but instead of fast zooms or slides, we should favour opacity fades.

https://css-tricks.com/empathetic-animation/#did-we-allow-users-to-opt-out

• [ ] Mobile drop-down slide in
• [ ] Mobile modal slide in