Remove italic style from blockquote

Italic blockquotes are hard to read and also we're not sure how it looks like for international characters. Although we remove this style now, users can apply italic style to the blockquote. They can select the specific _part_!

FYI, I didn't remove style from `cite`, but if you want to do it, delete `font-style: italic;` line from `article blockquote > cite` on the same file.