Add account usage logic (#1567)

---------

Co-authored-by: Yury Gargay <yury.gargay@gmail.com>

management/server/activity/event.go

@@ -8,12 +8,18 @@ const (
 	SystemInitiator = "sys"
 )
 
+// ActivityDescriber is an interface that describes an activity
+type ActivityDescriber interface {
+	StringCode() string
+	Message() string
+}
+
 // Event represents a network/system activity event.
 type Event struct {
 	// Timestamp of the event
 	Timestamp time.Time
 	// Activity that was performed during the event
-	Activity Activity
+	Activity ActivityDescriber
 	// ID of the event (can be empty, meaning that it wasn't yet generated)
 	ID uint64
 	// InitiatorID is the ID of an object that initiated the event (e.g., a user)