Post Commenting: implement design changes

The button has changed style; padding, font styles/size for description, commenter name styling and date.

The styling for button, description in comments section on post page revisited by William Morris-Julien, 

Implement the design as per the design files.

Nested Illustrator file is here - https://drive.google.com/open?id=1pbszuGPqr_XnI51H8gWN8jYE4Yct6tMj