Remove roots_get_avatar and replace with LESS

This commit replaces the roots_get_avatar function in favor of a LESS solution. Previously the function would sometimes not reliably match in the str_replace, for example when using plugins like Buddypress. 

I have added components/_comments and placed the LESS here, and modified main.less to include the partial.
The roots_get_avatar function has also been removed. 

This solution may only be a gap filler. For roots to be  framework agnostic the comment_walker and the comment template may need to be slightly modified, depending on how that will be accomplished.