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.

app/main/views/index.py

@@ -17,11 +17,7 @@ from app.main import main
 from app.main.forms import FieldWithNoneOption, SearchByNameForm
 from app.main.views.feedback import QUESTION_TICKET_TYPE
 from app.main.views.sub_navigation_dictionaries import features_nav, pricing_nav
-from app.utils import (
-    LETTER_SPECIFICATION_URL,
-    get_logo_cdn_domain,
-    user_is_logged_in,
-)
+from app.utils import get_logo_cdn_domain, user_is_logged_in
 
 
 @main.route('/')
@@ -273,7 +269,6 @@ def features_sms():
 def features_letters():
     return render_template(
         'views/features/letters.html',
-        letter_specification_url=LETTER_SPECIFICATION_URL,
         navigation_links=features_nav()
     )
 
@@ -348,3 +343,11 @@ def old_page_redirects():
         'main.old_integration_testing': 'main.integration_testing',
     }
     return redirect(url_for(redirects[request.endpoint]), code=301)
+
+
+@main.route('/docs/notify-pdf-letter-spec-latest.pdf')
+def letter_spec():
+    return redirect(
+        'https://docs.notifications.service.gov.uk'
+        '/documentation/images/notify-pdf-letter-spec-v2.4.pdf'
+    )