Record the actor on assign, archive and unarchive (#35)

Assign logged only the assignee (user_id), archive/unarchive logged nothing.
Migration 018 adds actor_user_id/actor_service_account_id to incident_events
for 'assigned'; archive/unarchive now log archived/unarchived events with the
caller via callerActorIDs. Timeline JSON gains actor_* fields; web timeline
renders them. Service accounts are still not assignable.

internal/models/incident.go

@@ -74,7 +74,8 @@ type Incident struct {
 // and is the only history this server keeps — alert rows are mutated in place.
 //
 // Type is one of: triggered, alert_added, alert_resolved, acknowledged,
-// unacknowledged, assigned, snoozed, unsnoozed, resolved, note. UserID and
+// unacknowledged, assigned, archived, unarchived, snoozed, unsnoozed,
+// resolved, note. UserID and
 // ServiceAccountID are mutually exclusive; both nil means the server acted
 // rather than any caller.
 type IncidentEvent struct {
@@ -92,6 +93,14 @@ type IncidentEvent struct {
 	ServiceAccountID   *int64  `json:"service_account_id,omitempty"`
 	ServiceAccountName *string `json:"service_account_name,omitempty"`
 
+	// Actor* name who performed an 'assigned' event, whose UserID is the
+	// assignee. Mutually exclusive; unset on every other event type and on
+	// assignments made before migration 018. See terdut-server#35.
+	ActorUserID             *int64  `json:"actor_user_id,omitempty"`
+	ActorUsername           *string `json:"actor_username,omitempty"`
+	ActorServiceAccountID   *int64  `json:"actor_service_account_id,omitempty"`
+	ActorServiceAccountName *string `json:"actor_service_account_name,omitempty"`
+
 	AlertID   *int64    `json:"alert_id,omitempty"`
 	Detail    *string   `json:"detail,omitempty"`
 	CreatedAt time.Time `json:"created_at"`