Support keyboard navigation (space bar, down arrow key)

[htroisi]

Most websites offer this interaction and we should support this too:

• pressing the space bar to scroll down by one page
• pressing the down arrow key to scroll down an arbitrary amount

[NickLanam]

After doing a little initial poking around, it seems that the element #gatsby-focus-wrapper is a parent of the scroll container and eats the keyboard events before they can do what they're supposed to do.

This seems to be a known issue with Gatsby: gatsbyjs/gatsby#29037
But there are workarounds there that we can try.