Custom Content Type: Improvements to the portfolio shortcode css

Responsive, If more then 2 columns use in mobile view display the grid
in 2 columns for readability.
Removed commas and columns from css implement via PHP.
Simplified the css, prefixed the css classes to minimized styling
conflicts.
Merge pull request #911 from Automattic/portfolio-shortcode-css

Custom Content Type: Improvements to the portfolio shortcode css