Add back-to-top link

[Ainali]

Based on https://codepen.io/chriscoyier/pen/NWmzZXo

Fixes #377

(If #380 is merged, the tests will pass.)

[RVA2869]

Wouldn't it be better to add the <a href="#top" class="to-top-link">Back to top</a> in layout.html?

[Ainali]

I considered that first but chose not to for two reasons:

1. Most important, it would be outside the <main> tag.
2. Less important, I think it is really only the country pages that require a lot of scrolling.

[RVA2869]

2. Less important, I think it is really only the country pages that require a lot of scrolling.

And the topics pages. ;-)