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/plugins/external_alert_check/status_definitions/status_external_alert.lua

@@ -0,0 +1,53 @@
+--
+-- (C) 2019 - ntop.org
+--
+
+local alert_consts = require("alert_consts")
+
+-- #################################################################
+
+local function formatIDSAlert(alert)
+   local signature = (alert and alert.signature)
+   local category = (alert and alert.category)
+   local signature_info = (signature and signature:split(" "));
+   local maker = (signature_info and table.remove(signature_info, 1))
+   local scope = (signature_info and table.remove(signature_info, 1))
+   local msg = (signature_info and table.concat(signature_info, " "))
+   if maker and alert_consts.ids_rule_maker[maker] then
+      maker = alert_consts.ids_rule_maker[maker]
+   end
+   return i18n("flow_details.ids_alert", { scope=scope, msg=msg, maker=maker })
+end
+
+-- #################################################################
+
+local function formatExternalAlert(status, flowstatus_info)
+   local res = i18n("alerts_dashboard.external_alert")
+
+   if not flowstatus_info then
+      return res
+   end
+
+   -- Available fields:
+   -- flowstatus_info.source (e.g. suricata)
+   -- flowstatus_info.severity_id (custom severity)
+   -- flowstatus_info.alert (alert metadata)
+
+   if flowstatus_info.source == "suricata" then
+      res = formatIDSAlert(flowstatus_info.alert)
+   end
+
+   return res
+end
+
+-- #################################################################
+
+return {
+  status_id = 21,
+  relevance = 0,
+  prio = 680,
+  alert_severity = alert_consts.alert_severities.error,
+  alert_type = alert_consts.alert_types.external_alert,
+  i18n_title = "alerts_dashboard.external_alert",
+  i18n_description = formatExternalAlert
+}