Fix issue #145

Made some teaks to [Tabzilla](https://www.mozilla.org/styleguide/websites/sandstone/tabzilla/) styles to match the appearance it has on [webmaker.org](https://webmaker.org/).

* Moved tabzilla styles to load before site styles (so they can be overridden)
* Added webmaker.org tabzilla image to repo (it has a slightly different shadow)
* Grouped all tabzilla hacks in the bottom of the `/css/site.css`.