Implement ntopng plugins

Plugins are a convenient way to group together related lua scripts.
Their primary use case is to group user scripts and their alert/status
definition.
The builtin ntopng user scripts and definitions are now
packed into plugins directories. In future, we will support loading of
user created plugins.
Plugins are loaded at startup into some runtime directories and then
used. Other changes provided by this commit include:

- Add sample flow logger plugin
- Initial support for system user scripts
- Rename edge to threshold
- Migrate system probes to user scripts/plugins
- Migrate scripts to more explicit alerts_api.checkThresholdAlert api

scripts/lua/modules/alert_utils.lua

@@ -118,6 +118,10 @@ function alertEngineRaw(granularity_id)
 end
 
 function alertEngine(v)
+   if(alert_consts.alerts_granularities[v] == nil) then
+      tprint(debug.traceback())
+   end
+
    return(alert_consts.alerts_granularities[v].granularity_id)
 end
 
@@ -138,6 +142,10 @@ function alertEngineDescription(v)
 end
 
 function granularity2sec(v)
+   if(alert_consts.alerts_granularities[v] == nil) then
+      tprint(debug.traceback())
+   end
+
   return(alert_consts.alerts_granularities[v].granularity_seconds)
 end
 
@@ -780,28 +788,29 @@ end
 -- #################################
 
 local function printProbesTab(entity_probes, entity_type, entity_value, page_name, page_params, alt_name, options)
-   local system_scripts = require("system_scripts_utils")
+   -- TODO migrate probes
+   --~ local system_scripts = require("system_scripts_utils")
 
-   if #entity_probes > 0 then
-      print[[
-   <br>
-   <table class="table table-bordered table-striped">
-     <tr>
-       <th width="10%">]] print(i18n("system_stats.probe")) print[[</th>
-       <th width="25%">]] print(i18n("system_stats.probe_config")) print[[</th>
-     </tr>]]
+   --~ if #entity_probes > 0 then
+      --~ print[[
+   --~ <br>
+   --~ <table class="table table-bordered table-striped">
+     --~ <tr>
+       --~ <th width="10%">]] print(i18n("system_stats.probe")) print[[</th>
+       --~ <th width="25%">]] print(i18n("system_stats.probe_config")) print[[</th>
+     --~ </tr>]]
 
-      for _, probe in ipairs(entity_probes) do
-      print[[
-     <tr>
-       <td>]] print(probe["probe"]["name"]) print[[</td>
-       <td><a href="]] print(probe["config"]["url"]) print[["><i class="fa fa-cog" aria-hidden="true"></i></a></td>
-     </tr>]]
+      --~ for _, probe in ipairs(entity_probes) do
+      --~ print[[
+     --~ <tr>
+       --~ <td>]] print(probe["probe"]["name"]) print[[</td>
+       --~ <td><a href="]] print(probe["config"]["url"]) print[["><i class="fa fa-cog" aria-hidden="true"></i></a></td>
+     --~ </tr>]]
 	 
-      end
+      --~ end
 
-      print[[</table>]]
-   end
+      --~ print[[</table>]]
+   --~ end
 end
 
 -- #################################
@@ -1267,12 +1276,14 @@ function printAlertTables(entity_type, alert_source, page_name, page_params, alt
    if(tab == nil) then tab = "config" end
    local is_alert_list_tab = ((tab == "alert_list") or (tab == "past_alert_list") or (tab == "flow_alert_list"))
 
+   --[[ TODO migrate probes
    local system_scripts = require("system_scripts_utils")
    local entity_probes = system_scripts.getEntityProbes(entity_type, alert_source)
 
    if #entity_probes > 0 then
       printTab("probes", i18n("system_stats.probes"), tab)
    end
+   ]]
 
    printTab("config", '<i class="fa fa-cog" aria-hidden="true"></i> ' .. i18n("traffic_recording.settings"), tab)
 
@@ -2397,6 +2408,10 @@ function formatAlertMessage(ifid, alert)
     end
   end
 
+  if(type(msg) == "table") then
+   return("")
+  end
+
   return(msg)
 end