Overlay truly over lays

The open overlay button is no longer on to of the overlay. When the
overlay is open, it sits on top of everything else now.
Moved overlay button to sidebar

Ok, so first things first, I kinda got it wrong in the last commit: the
overlay doesn't sit on top of EVERYTHING when it opens. Only the map is
beneath the overlay. All leaflet controls (sidebar, layers, weather,
etc.) are still accessible/interactable when overlay is up.

Anywayyy, the "open overlay" button has been moved into the "settings"
tab of the sidebar and works as expected/intended. It was literally just
a wholesale copy/paste from one part of the source to another.