Refactor to move preview logic to API

* Moved the notifications code to go to admin to get the the template

preview document rather than go to template preview.

This will remove the logic from admin and place it in api so it is
easier to expand on later when there are precompiled PDFs

* Added some error handling if API returns an API error.

Caught the error and displayed an error PNG so it is obvious something
failed. Currently it displayed a thumbnail of a png over the top of the
loading page, and therefore it wasn't obvious of the state.

tests/app/main/views/test_notifications.py

@@ -1,9 +1,11 @@
+import base64
 from functools import partial
+from unittest.mock import mock_open
 
 import pytest
 from flask import url_for
 from freezegun import freeze_time
-from notifications_utils.template import LetterImageTemplate
+from notifications_python_client.errors import APIError
 from tests.conftest import (
     SERVICE_ONE_ID,
     mock_get_notification,
@@ -163,9 +165,11 @@ def test_should_show_image_of_letter_notification(
 
     mock_get_notification(mocker, fake_uuid, template_type='letter')
 
-    mocked_preview = mocker.patch(
-        'app.main.views.templates.TemplatePreview.from_utils_template',
-        return_value='foo'
+    mocker.patch(
+        'app.notify_client.notification_api_client.NotificationApiClient.get',
+        return_value={
+            'content': base64.b64encode(b'foo').decode('utf-8')
+        }
     )
 
     response = logged_in_client.get(url_for(
@@ -177,8 +181,32 @@ def test_should_show_image_of_letter_notification(
 
     assert response.status_code == 200
     assert response.get_data(as_text=True) == 'foo'
-    assert isinstance(mocked_preview.call_args[0][0], LetterImageTemplate)
-    assert mocked_preview.call_args[0][1] == filetype
+
+
+def test_should_show_preview_error_image_letter_notification_on_preview_error(
+    logged_in_client,
+    fake_uuid,
+    mocker,
+):
+
+    mock_get_notification(mocker, fake_uuid, template_type='letter')
+
+    mocker.patch(
+        'app.notify_client.notification_api_client.NotificationApiClient.get',
+        side_effect=APIError
+    )
+
+    mocker.patch("builtins.open", mock_open(read_data="preview error image"))
+
+    response = logged_in_client.get(url_for(
+        'main.view_letter_notification_as_preview',
+        service_id=SERVICE_ONE_ID,
+        notification_id=fake_uuid,
+        filetype='png'
+    ))
+
+    assert response.status_code == 200
+    assert response.get_data(as_text=True) == 'preview error image'
 
 
 def test_should_404_for_unknown_extension(