Fix header icon alignment

[renaudleo]

As header icons are wrapped in <span> tags, every icon for wich that tag starts on a new line gets moved a bit to the right because of the resulting space. The close button and readability options button are two examples of this as of now.

We could probably get away with some kind of spaceless helper, but I'd rather find a way to remove the span tag entirely. Will have to look into it.

[j15e]

font-size 0?

[renaudleo]

Won't work that easily if we want to keep using em as padding unit. Ex: 1.3em * 0 = 0 :(

[j15e]

two containers?