WIKI: allow folding folder contents in sidebar.

Currently, the sidebar shows all wiki pages (first x of them).

There's no option to "fold" a folder.

Instead, since the folder support is already implemented, we can show all pages in the sidebar, with each folder's contents collapsed.

What is it?

A suggestion to change the sidebar to allow folding folders' contents and showing all pages without needing to load another page.

Why should someone use this feature?

It can be very useful to navige huge WIKIs

What is the underlying (business) problem?

Huge WIKIs are hard to navigate

How do you use this feature?

Navigate down the tree until I find the document I want.

To help navigate huge WIKIs

Make sure these are completed before closing the issue, with a link to the relevant commit.