Added a footer to the website for spacing.

[mwhittaker]

Before this PR, our website had no padding at the bottom. This meant that the end of page was at the very bottom of the screen. This made reading the end of a page---like the end of a blog post---a little irritating. This PR adds a footer to the bottom of every page. In the future, we may want to add some contents to the footer, but for now it exists just to add a bit of space.