Display postage everywhere we display a letter

To avoid the problem of having confusing defaults, the postage is now
set explicitly on every template.

Putting the postage ‘inside’ the letter template makes the interaction
for changing it consistent with how other parts of the template are
added.

Plus everyone loves skeumorphism.

app/utils.py

@@ -331,7 +331,8 @@ def get_template(
                 template,
                 image_url=letter_preview_url,
                 page_count=int(page_count),
-                contact_block=template['reply_to_text']
+                contact_block=template['reply_to_text'],
+                postage=template.get('postage') or service.postage,
             )
         else:
             return LetterPreviewTemplate(