Add Zoom To World Control

Add a leaflet control to the map.

• This control should use an icon of the world
  • Use material design icons https://material.io/resources/icons/?search=earth&icon=public&style=baseline
• The control should zoom to the default extents (world wide)
• Position control in upper-left-hand corner of map, below the +/- zoom controls
• If the icon is still legible it would be nice if the control were the same size (width) as the +/- zoom controls