Add scrollbar to #page-nav

[OddKristensen]

Took a quick stab at issue 706. There are many ways to solve this, but simply adding a height and a scrollbar on vertical overflow is the easiest solution that does not require additional changes to the DOM. Again, with http4s's changelog as an example it will look like this:

I picked height: 80% because it still looks somewhat ok an really short screens. I found that 90% was too cramped when the screen was getting really short.

With height: 90% it looks like the screenshot on the bottom, where you cannot actually scroll all the way down.

Compared that wight height: 80%:

[jenshalm]

Welcome, and thanks for preparing this PR. I wasn't aware of this issue.

Please verify the one suggested change, as the proposed solution breaks pages with short page navigation boxes.