improvement on service_map page

The graph page has been set as the first page wehn a user navigates to
the service map page. When a user double clicks a node then the map will
be filtered with only the edges linked to the node. The flush data and
download buttons have been restored.

scripts/lua/service_map_handler.lua

@@ -0,0 +1,27 @@
+--
+-- (C) 2020 - ntop.org
+--
+
+local dirs = ntop.getDirs()
+package.path = dirs.installdir .. "/scripts/lua/modules/?.lua;" .. package.path
+
+require "lua_utils"
+local json = require("dkjson")
+local ifs = interface.getStats()
+
+sendHTTPHeader('application/json')
+
+local ifid = ifs.id
+local REDIS_KEY = string.format("ntopng.prefs.service_map.%d.graph", ifid)
+
+-- prepare response for the request
+local res = {}
+-- get the node position
+local graph_view = _POST['JSON']
+
+ntop.setPref(REDIS_KEY, graph_view)
+
+res.success = true
+res.message = "The graph view has been saved!"
+
+print(json.encode(res))