Add a redirect for the letter specification

This way we have a URL we can give people that always points to the
latest version of the spec.

And it makes our code more Flask-idiomatic to be using `url_for` to be
generating a URL, rather than passing around a constant.

tests/app/main/views/test_index.py

@@ -254,3 +254,21 @@ def test_letter_template_preview_headers(
     )
 
     assert response.headers.get('X-Frame-Options') == 'SAMEORIGIN'
+
+
+def test_letter_spec_redirect(client_request):
+    expected_url = (
+        'https://docs.notifications.service.gov.uk'
+        '/documentation/images/notify-pdf-letter-spec-v2.4.pdf'
+    )
+    client_request.get(
+        'main.letter_spec',
+        _expected_status=302,
+        _expected_redirect=expected_url,
+    )
+    client_request.logout()
+    client_request.get(
+        'main.letter_spec',
+        _expected_status=302,
+        _expected_redirect=expected_url,
+    )