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.
Fixed admin menu bug on mobile
Merge pull request #911 from Automattic/portfolio-shortcode-css

Custom Content Type: Improvements to the portfolio shortcode css