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/s3_client/test_s3_letter_upload_client.py

@@ -0,0 +1,17 @@
+from flask import current_app
+
+from app.s3_client.s3_letter_upload_client import upload_letter_to_s3
+
+
+def test_upload_letter_to_s3(mocker):
+    s3_mock = mocker.patch('app.s3_client.s3_letter_upload_client.utils_s3upload')
+
+    upload_letter_to_s3('pdf_data', 'service_id/upload_id.pdf', 'valid')
+
+    s3_mock.assert_called_once_with(
+        bucket_name=current_app.config['TRANSIENT_UPLOADED_LETTERS'],
+        file_location='service_id/upload_id.pdf',
+        filedata='pdf_data',
+        metadata={'status': 'valid'},
+        region=current_app.config['AWS_REGION']
+    )