XWIKI-11498-Blog: need a space character between icons of blog-entry-too...

...lbox
@sdumitriu is suggesting to use [relative length units](https://developer.mozilla.org/en-US/docs/Web/CSS/length#Relative_length_units). While I agree this is generally a best practice, I've seen that Bootstrap is using absolute units in some places, e.g. the navigation bar.