Bug 726853: It's Tabzilla Time for AMO

Only the impala theme template and styles.

Works locally for the pages I manually checked.

[comments below from @ngokevin]:

This patch changes the AMO/devhub gray tab to be the new slide-down tab global across Mozilla sites (tabzilla).

[Requirements](http://bedrock.readthedocs.org/en/latest/tabzilla.html#requirements)

- Adds required CSS/JS files.
- Adds an outer-wrapper ```div``` to the base Zamboni and base impala templates.
- Moves the ```background``` CSS rule from ```html``` to the wrapper so the background slides down with the page
- Moves 2px ```border-top``` rule from the ```body``` to the wrapper as per requirements

![screen shot 2013-10-08 at 7 39 02 pm](https://f.cloud.github.com/assets/674727/1294709/fa0fa992-308b-11e3-9533-4f98bccc4f07.png)