Sanitise uploaded letters and store in S3

This sanitises uploaded letters and stores the sanitised result in S3
with if it passes validation or the original PDF in S3 if validation
fails. A metadata value of 'status' is set to either 'valid' or
'invalid'.

tests/app/test_template_previews.py

@@ -4,7 +4,11 @@ from unittest.mock import Mock
 import pytest
 from notifications_utils.template import LetterPreviewTemplate
 
-from app.template_previews import TemplatePreview, get_page_count_for_letter
+from app.template_previews import (
+    TemplatePreview,
+    get_page_count_for_letter,
+    sanitise_letter,
+)
 
 
 @pytest.mark.parametrize('partial_call, expected_page_argument', [
@@ -119,3 +123,15 @@ def test_from_example_template_makes_request(mocker):
               'filename': filename,
               'letter_contact_block': None}
     )
+
+
+def test_sanitise_letter_calls_template_preview_sanitise_endoint_with_file(mocker):
+    request_mock = mocker.patch('app.template_previews.requests.post')
+
+    sanitise_letter('pdf_data')
+
+    request_mock.assert_called_once_with(
+        'http://localhost:9999/precompiled/sanitise',
+        headers={'Authorization': 'Token my-secret-key'},
+        data='pdf_data'
+    )