Collapse/expand elements in the table of contents

[essoen]

The table of contents on the left grows quite long when an article grows. Over time it's hard to find anything there. See for instance this image from the Security Engineering article:

tcoI thought it would be nice to collapse and expand the sections in the table of contents:

• You have a little arrow on the left side of each element in the TOC, Press it, and it'll show the elements it has, and the arrow will point down to indicate it has been expanded. Something like this (just nicer):

• Articles can be automatically collapsed when you open them, so that the user can explore the content, without being overwhelmed if there's a lot to read about.