Code styling fixes:

-   For all <pre> blocks, allow horizontal scrolling
    instead of wrapping text.
-   For side-by-side element in tutorial, keep code
    block from stretching vertically.
-   Use link color for <a><code>. Nesting <code><a>
    already works, but is error-prone; this just ensures
    we use the right color either way.
Removed commented-out property.
Merge pull request #532 from arthurevans/tutorial-codeformat

Code styling fixes