Add support for collapsing/expanding the diff of a page from the extension diff details

When the extension diff has many modified pages it's quite difficult to navigate the details as you have to scroll a lot. Would be great if we could:

• collapse/expand the diff of a page once we have read it
• emphasize the start of a page diff (the heading) a bit more so that it's easier to find a page and to spot when a new page diff stats (e.g. when you scroll down).