Improve the content visibility of the Wysiwyg adding a 5px margin left and right

To reproduce:
1- Create any wiki page
2- Edit it using the WYSIWYG
3- Add some texte and look on the side of the wysiwug editor

See screenshot 1 for windowed mode
See screenshot 2 for fullscreen mode

We may add 5px margin left and right to improve visibility for both mode in order to visually split the text from the border of the element.