Collapsible sidebar

[panda01]

Added a directive for collapsing sidebars. Closes #3037.

[jthomassie]

@panda01 this looks good to me functionally. Stylistically (is that a word?), I think the width of the 'sidebar-collapser' feels a little tight and covers up the right edge of the 'sidebar-list'. Maybe it could be styled like the tab to open and close the 'spy panel'.
See what @rashidkpc thinks.

[stormpython]

@panda01 This LGTM, but I wonder when the screen size is reduced whether there is a need for the collapsible sidebar anymore. For example:

In this situation the side bar is already collapsed. So, I do not think there is a need for the collapsible directive applied to the sidebar. See below.

But it's certainly not a blocker. May want to ask @rashidkpc or @spalger.

[panda01]

Smooth thanks for the review, I put a solution that I believe will work!

[simianhacker]

LGTM. Feel free to merge this.