Fix for open redirect - identified in Issue 2429 (#2500)

* Issue 2429 indicates the possiblity of an open redirect

The 404 processing ends up redirecting a request with multiple path
slashes to that site, i.e.

https://my-site//shedcode.co.uk will redirect to https://shedcode.co.uk

This commit uses a regular expression to remove the multiple leading
slashes before redirecting.

tests/test_custom_pages.py

@@ -97,3 +97,9 @@ def test_custom_route_pattern_404(custom_pages_client):
     assert response.status == 404
     assert "<h1>Error 404</h1>" in response.text
     assert ">Oh no</" in response.text
+
+
+def test_custom_route_pattern_with_slash_slash_302(custom_pages_client):
+    response = custom_pages_client.get("//nastyOpenRedirect/")
+    assert response.status == 302
+    assert response.headers["location"] == "/nastyOpenRedirect"