Website UI Changes

Hello!

I noticed some small things on the website that I wanted to take a shot at clearing up.

1) The link in the header to "Github" should read "GitHub" to match their preferred capitalization preferences. Easy enough!

2) The Tabzilla bar wasn't correctly pulling down, according to the behavior on [the Mozilla homepage](http://www.mozilla.org). You can see from the diffs what I did to get that working.

Let me know if anything needs to be changed/could have been done better!

:bear: 

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Edit: This blows away the `position: fixed` property of the navbar. I'm not sure if there's an elegant way to have both the fixed nav and the tabzilla bar above it.