making raw source display better

* Make the code fill up the full width of the page (no massive whitespace on the left)
* Move the code down to make it not intersect the logo
* Set a min-width and remove the max-width so that the code doesn't scroll internally, but instead scrolls the page, meaning horizontal scroll bars are always available
* Set overflow to actually overflow, just to be sure

Fixes #15891
auto merge of #16066 : Gankro/rust/code-display-fix, r=alexcrichton

Summary of changes:

* Make the code fill up the full width of the page (no massive whitespace on the left)
* Move the code down to make it not intersect the logo
* Set a min-width and remove the max-width so that the code doesn't scroll internally, but instead scrolls the page, meaning horizontal scroll bars are always available
* Set overflow to actually overflow, just to be sure

Fixes #15891

![two-screen](https://cloud.githubusercontent.com/assets/1136864/3730216/3315a45e-16cc-11e4-9268-4ebf65583f20.png)
![wide-screen](https://cloud.githubusercontent.com/assets/1136864/3730217/33301302-16cc-11e4-9aa9-6c28e1203999.png)