Refine Mobile Styles

* Header has fixed position
* Update search bar placeholder text and styles
* Move "Repo" link into account list
* Decrease font size on mobile

https://trello.com/c/8lxiMPx3

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![hound-mobile-scroll](https://cloud.githubusercontent.com/assets/1174461/3884502/3e0c436a-21b0-11e4-9ce0-9426d28d969f.gif)

I can get rid of `@include placeholder`. `&[placeholder]` is sufficient with what I'm trying to do.

Good :eyes: