Sidebar overlaps content when screen is below 1200px.

When screen is below 1200px, the sidebar overlaps.
When screen is above 1200px, the sidebar pushes content out.
z-index change to make sure hamburger stays on top.
Fixes #12717
![screensize](/uploads/9a21fa06d583a49d6ebbf1ada34c6792/screensize.gif)

![screensize-small](/uploads/7c25f46e962248a40840562a01c83f8f/screensize-small.gif)

Also sorry I couldn't get the collapse button in the screen cap. It's there.

See merge request !2620