add handler for 403s

fixes #3104

bookwyrm/views/__init__.py

@@ -167,3 +167,4 @@ from .annual_summary import (
     summary_revoke_key,
 )
 from .server_error import server_error
+from .permission_required import permission_required

bookwyrm/views/permission_required.py

@@ -0,0 +1,8 @@
+"""custom 403 handler to enable context processors"""
+from django.template.response import TemplateResponse
+
+
+def permission_required(request, exception):  # pylint: disable=unused-argument
+    """permission denied page"""
+
+    return TemplateResponse(request, "403.html")