Add direct animation control below nav

[joemull]

Fixes #249.

[ajrbyers]

I commented in Discord and then removed it. From what I can see C39 and SCR40 are sufficient for this but if we're going beyond that this is fine. I feel like it detracts from the page design, though.

[StephDriver]

I'm with Andy that it sticks out on the page. I find it cluttering and would love a way to dismiss it. I wonder whether, if a user already has reduced-motion set up, we could not show this? Or whether there would be a minimise or unstick option once it has been interacted with.

For me, as a user who does have reduced motion active anyway, there's little likelihood I'd be turning it on anywhere there wouldn't be play/pause buttons anyway.

However, It does improve accessibility, and eventually we'll have a whole accessibility page added like we are doing for the other themes, and then it will have a home with other settings. For an interim solution, I don't mind this.

I have put in a comment about the changes to templates/custom/open-access-movement.html which I think may be from another PR rather than this one? I can't see how they affect the rest of this change. That is why I'm sending it back to you rather than merging it myself.

More generally, I approve the change as a good interim improvement.

[StephDriver]

I don't understand this change. I can't find "step" among any of the other changes.