Add system activity tracking and event store (#636)

This PR adds system activity tracking. 
The management service records events like 
add/remove peer,  group, rule, route, etc.

The activity events are stored in the SQLite event store
and can be queried by the HTTP API.

management/server/event.go

@@ -0,0 +1,33 @@
+package server
+
+import (
+	"fmt"
+	"github.com/netbirdio/netbird/management/server/activity"
+)
+
+// GetEvents returns a list of activity events of an account
+func (am *DefaultAccountManager) GetEvents(accountID, userID string) ([]*activity.Event, error) {
+	events, err := am.eventStore.Get(accountID, 0, 10000, true)
+	if err != nil {
+		return nil, err
+	}
+
+	// this is a workaround for duplicate activity.UserJoined events that might occur when a user redeems invite.
+	// we will need to find a better way to handle this.
+	filtered := make([]*activity.Event, 0)
+	dups := make(map[string]struct{})
+	for _, event := range events {
+		if event.Activity == activity.UserJoined {
+			key := event.TargetID + event.InitiatorID + event.AccountID + fmt.Sprint(event.Activity)
+			_, duplicate := dups[key]
+			if duplicate {
+				continue
+			} else {
+				dups[key] = struct{}{}
+			}
+		}
+		filtered = append(filtered, event)
+	}
+
+	return filtered, nil
+}