Added a 'Draft' watermark to the seasonal map view

This stamps a 'Draft' image over the map. It is implemented as a Leaflet map control since trying to overlay an image messed up the interactions of the underlying map controls. Its position is fixed relative to the bottom left of the map - no chance to make it always remain central, so it is possible for it to disappear off the top or right on very small screens.

This is what it looks like on my pc: