Show more button template

Moved facia show more button into template file.

I was thinking about moving the 'live blog' and 'comments' show more buttons so they will be using the same template. But they use so many different styles, data attributes etc that I am not sure if it's worth it. It will be nice if these show more buttons could follow more similar pattern as they are theoretically doing the same thing but at this point it seems to need a bigger refactoring.

I did a small change in `container-fc-show-more.js` on line [34](https://github.com/guardian/frontend/compare/show-more-button-template?expand=1#diff-fb1e253691f530691d810e6aefec9413R34) so it will access DOM just once.