Focus search on open + add aria attributes

This is ready for review. A couple notes:

• Both mobile and desktop search buttons now toggle aria-expanded
• After opening the search drawer (both mobile and desktop), the search input will now focus.
• Open / close animation is much smoother now. There was some weirdness before that was being solved with a transition delay on the search form.
• I added testing instructions to the IS

I reviewed the code and tested everything locally and it all works as expected. Thanks!

Merging this. It will go with the next deployment to the site.