Update utils with to make letters unclickable

Brings in:
- [ ] https://github.com/alphagov/notifications-utils/pull/159

Means:
- renaming a bunch of things
- removing any CSS that targeted the link, and making it target the
  containing element too

app/utils.py

@@ -19,7 +19,7 @@ from flask_login import current_user
 from notifications_utils.template import (
     SMSPreviewTemplate,
     EmailPreviewTemplate,
-    LetterPDFLinkTemplate,
+    LetterImageTemplate,
     LetterPreviewTemplate,
 )
 
@@ -274,9 +274,9 @@ def get_template(
         )
     if 'letter' == template['template_type']:
         if letter_preview_url:
-            return LetterPDFLinkTemplate(
+            return LetterImageTemplate(
                 template,
-                preview_url=letter_preview_url,
+                image_url=letter_preview_url,
                 page_count=int(page_count),
             )
         else: