Refactors alerts and flow statuses in builtin and plugins

Implements #3811
Implements #3812

Refactors builtin alerts

Refactors builtin flow statuses

scripts/lua/modules/alert_definitions/alert_slow_purge.lua

@@ -0,0 +1,25 @@
+--
+-- (C) 2019-20 - ntop.org
+--
+
+local alert_keys = require "alert_keys"
+
+local function formatSlowPurge(ifid, alert, threshold_info)
+  local alert_consts = require("alert_consts")
+  local entity = alert_consts.formatAlertEntity(ifid, alert_consts.alertEntityRaw(alert["alert_entity"]), alert["alert_entity_val"])
+  local max_idle_perc = threshold_info.threshold or 0
+
+  return(i18n("alert_messages.slow_purge", {
+    iface = entity, max_idle = max_idle_perc,
+    url = ntop.getHttpPrefix() .. "/lua/if_stats.lua?ifid=" .. ifid .. "&page=internals",
+  }))
+end
+
+-- #######################################################
+
+return {
+  alert_key = alert_keys.ntopng.alert_slow_purge,
+  i18n_title = "alerts_dashboard.slow_purge",
+  icon = "fas fa-exclamation",
+  i18n_description = formatSlowPurge,
+}