LPS-51699 Cover Image for Blogs

cc/ @ipeychev 

Hi Nate, 
here is the PR with all the changes you suggested in https://github.com/natecavanaugh/liferay-portal/pull/3009. The only comment I was not sure was about the attribute "color" that was  in `.progress-wrapper > p`. That's the color defined by the design team and does not match with any inherited class, so we need it. I moved it to the parent class. Let me know if that was not what you meant. 

Thanks!