Add style for markdown footnotes

We might want to consider styling with markdown footnotes. I can envision a thin line separator, maybe a slightly smaller font (like real footnotes). We might also consider a different backref icon (or I would even suggest not using an icon, just the actual unicode return ↩ character, which would be more subtle).

Once https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24168 is merged, we will have the proper classes wrapping the footnote elements:

1. The entire footnote section is wrapped in <section class="footnotes">
2. The back reference link has the footnote-backref class
3. The superscripted reference has the footnot-ref class

I'm creating a footnote[^1] here as an example of how they are currently styled.

[^1]: This is my example footnote

I'm creating a footnote¹ here as an example of how they are currently styled.

1. This is my example footnote ↩