change letter previews to be in the no_cookies blueprint

this blueprint should be applied to every endpoint that is loaded async
(as in via a src tag on an img, iframe, etc)

tests/app/main/views/test_index.py

@@ -226,7 +226,7 @@ def test_letter_template_preview_links_to_the_correct_image(
     image_link = page.find('img')['src']
 
     assert image_link == url_for(
-        'main.letter_branding_preview_image',
+        'no_cookie.letter_branding_preview_image',
         filename=filename,
         page=1
     )