Dev: add prettyprint class to every pre and line numbers to every second

Makes it easier to test the different styles.
Update Codekit config
Add CSSComb config file
Sass: Add footer styling
Use spaces instead of tabs
Use rem instead of px in the prettyprint selector
Add datalist and multiple select to the test page
Add the .help class in addition to .required for labels
spin is not a Sass function
Sass: Forms
Move the invalid input declaration to a generic input selector
Style file inputs and the file input button