[SECURITY] Centralize framebreaking logic from payment plugins to core (CVE-2026-13602)

- Add central framebreaker page via safelink helper
- Update paypal, paypal2 and stripe plugins to use central framebreaker
- Add CSP header to cookies.html

---------

Co-authored-by: Mira Weller <weller@pretix.eu>

src/pretix/plugins/paypal2/views.py

@@ -36,13 +36,10 @@ import logging
 from decimal import Decimal
 
 from django.contrib import messages
-from django.core import signing
 from django.core.cache import cache
 from django.db import transaction
 from django.db.models import Sum
-from django.http import (
-    Http404, HttpResponse, HttpResponseBadRequest, JsonResponse,
-)
+from django.http import Http404, HttpResponse, JsonResponse
 from django.shortcuts import get_object_or_404, redirect, render
 from django.urls import reverse
 from django.utils.decorators import method_decorator
@@ -104,21 +101,6 @@ class PaypalOrderView:
         }) + ('?paid=yes' if self.order.status == Order.STATUS_PAID else ''))
 
 
-@xframe_options_exempt
-def redirect_view(request, *args, **kwargs):
-    signer = signing.Signer(salt='safe-redirect')
-    try:
-        url = signer.unsign(request.GET.get('url', ''))
-    except signing.BadSignature:
-        return HttpResponseBadRequest('Invalid parameter')
-
-    r = render(request, 'pretixplugins/paypal2/redirect.html', {
-        'url': url,
-    })
-    r._csp_ignore = True
-    return r
-
-
 @method_decorator(csrf_exempt, name='dispatch')
 @method_decorator(xframe_options_exempt, 'dispatch')
 class XHRView(View):