Add some missing files from previous commits

platform/session.go

@@ -0,0 +1,22 @@
+package platform
+
+import (
+	"context"
+	"fmt"
+)
+
+type session struct {
+	Impersonating      *User
+	NotificationCounts notificationCounts
+}
+
+func SessionCurrent(ctx context.Context, user User) (*session, error) {
+	counts, err := NotificationCountsForUser(ctx, user)
+	if err != nil {
+		return nil, fmt.Errorf("get notifications: %w", err)
+	}
+	return &session{
+		Impersonating:      nil,
+		NotificationCounts: *counts,
+	}, nil
+}