Don’t repeat the letter spec URL in the code

We change this URL fairly frequently because we bump the version number.
Let’s make it easier to change by only defining it once.

app/main/views/index.py

@@ -17,7 +17,11 @@ 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 get_logo_cdn_domain, user_is_logged_in
+from app.utils import (
+    LETTER_SPECIFICATION_URL,
+    get_logo_cdn_domain,
+    user_is_logged_in,
+)
 
 
 @main.route('/')
@@ -269,6 +273,7 @@ def features_sms():
 def features_letters():
     return render_template(
         'views/features/letters.html',
+        letter_specification_url=LETTER_SPECIFICATION_URL,
         navigation_links=features_nav()
     )