Fix bug with letter job CSV downloads

We were throwing an exception when instantiating a LetterImageTemplate
as we weren't giving it all the arguments it needed.

 Now we give it all the correct parameters and add a
test for the method. Ideally we would add a unit test for the flask
route for downloading a letter job CSV (which is currently lacking) but
I did the minimal to be confident I've fixed the bug as I think this
whole code may be fresh for a bit of a rewrite according to Chris.

Original error:
```
File "/Users/davidmcdonald/.virtualenvs/notifications-admin/lib/python3.6/site-packages/notifications_utils/template.py", line 669, in __init__
    raise TypeError('image_url is required')
TypeError: image_url is required
```

app/utils.py

@@ -157,7 +157,9 @@ def get_sample_template(template_type):
     if template_type == 'sms':
         return SMSPreviewTemplate({'content': 'any', 'template_type': 'sms'})
     if template_type == 'letter':
-        return LetterImageTemplate({'content': 'any', 'subject': '', 'template_type': 'letter'})
+        return LetterImageTemplate(
+            {'content': 'any', 'subject': '', 'template_type': 'letter'}, postage='second', image_url='x', page_count=1
+        )
 
 
 def generate_notifications_csv(**kwargs):