Map legend must use style title if available

[giohappy]

Now that style titles have been fixed (#7405) we need to use them (if available).
MapLayer.get_legend must return the title as the dictionay keys, if it's available, otherwise it will use the style name.

geonode/geonode/maps/models.py

Lines 583 to 595 in 4e37099

	try:
	layer_params = json.loads(self.layer_params)
	capability = layer_params.get('capability', {})
	# Use '' to represent default layer style
	style_name = capability.get('style', '')
	layer_obj = Layer.objects.filter(alternate=self.name).first()
	if ':' in style_name:
	style_name = style_name.split(':')[1]
	elif layer_obj.default_style:
	style_name = layer_obj.default_style.name
	href = layer_obj.get_legend_url(style_name=style_name)
	return {style_name: href}