[mathjax] Add scrollbar inside dialog for mathjax preview wider than viewport

[kkrzton]

Noticed when fixing #13612. One think is proposed fix for #13612 which repositions the dialog after rendering mathjax preview so it's always centered (​see detailed description).

The second think is sometimes mathajx preview can be so wide it does not fit inside the viewport even when repositioning dialog. To see how it looks, simply paste this formula inside mathjax dialog:

x = {-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a} x = {-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}{-b \pm \sqrt{b^2-4ac} \over 2a}

Probably not many people use such long and complicated formulas. However adding a scrollbar inside dialog (to enable mathjax preview scrolling) and restricting its maximum width to a viewport width could solve this issue.