v4.0.0

Major enhancements

• Replace the modified_date variable of posts with the last_modified_at variable since the later variable would also be considered by jekyll-seo-tag.
• Delete the $display-font-family SCSS variable.

Minor enhancements

• Adjust fonts.
• Show the location of the author on the sidebar.
• Show the comment count of each post on the homepage.
• Slightly adjust the layout.