Add a the Leaflet.Fullscreen plugin as a default with Leaflet preview 

__What does this PR do?__

The Leaflet.fullscreen plugin is one of the coolest plugins out there, why don't we make it a default on the layer_detail view?  This just adds an extra Javascript call to enable the control using an extra key in the context_dict for the view that checks it is present as a plugin with the name 'leaflet-fullscreen' in settings.py.

I guess a caveat is that the control won't work in some versions of IE.