849: Accessible accordion attributes

[dcmouyard]

Fixes #849. This PR:

• Removes the unnecessary attributes on accordion sections
• Updates the helper slide functions to account for prefers-reduced-motion

[tomealter]

@dcmouyard I pushed up some updates to the accordion JS to trim some fat and make things a little more consistent.

[dcmouyard]

@tomealter Your changes look good to me. I just pushed up a tweak, though, that:

• Renames step_list variable to is_step_list so it’s more clear that it’s a boolean.
• Only output the step list markup if it’s a step list.