When contents of a panel is too big the North Toolbar breaks

[clarkepaul]

In the following case the image within the history of a page should be limited to max-width 100%, as it is not constrained the North Toolbar breaks into the preview.

I would think that we need to fix the over flowing file but also it would be good to see if we can fix it even if there was overflowing content within the body of the panel. Another case of this happening is within the comments module silverstripe/silverstripe-comments#243.

[robbieaverill]

Fixed in #527