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):

tests/app/test_utils.py

@@ -7,6 +7,7 @@ import pytest
 from bs4 import BeautifulSoup
 from flask import url_for
 from freezegun import freeze_time
+from notifications_utils.template import Template
 
 from app import format_datetime_relative
 from app.utils import (
@@ -18,6 +19,7 @@ from app.utils import (
     get_letter_printing_statement,
     get_letter_validation_error,
     get_logo_cdn_domain,
+    get_sample_template,
     is_less_than_90_days_ago,
     printing_today_or_tomorrow,
 )
@@ -576,3 +578,9 @@ def test_get_letter_validation_error_for_known_errors(
 @freeze_time('2020-02-14T12:00:00')
 def test_is_less_than_90_days_ago(date_from_db, expected_result):
     assert is_less_than_90_days_ago(date_from_db) == expected_result
+
+
+@pytest.mark.parametrize("template_type", ["sms", "letter", "email"])
+def test_get_sample_template_returns_template(template_type):
+    template = get_sample_template(template_type)
+    assert isinstance(template, Template)