Add newsletter thank you CSS animation

* Added CSS animation for newsletter "thank you" state.
* Fixed up some page-specific CSS styles for the updated newsletter form.

Might need a few more tweaks here and there on some pages, but from initial testing it seems to work pretty well of different pages / browsers / viewport sizes.

Good catch, thanks. It could be done in CSS yeah, but not as easily degradable for older browsers. I'll see how fiddly it might be.