Bug 808761 - Increase hit target for back btn 50px

Increase hit target of back button to 50px (5rem).

Had to tweak a few properties for `header:first-child > button`
and `header:first-child > a`, since the original implementation
relied on centered positioning. Also removed moz-focus-inner padding
and border from button. This removes the 1px offset that buttons
have as compared to links.