Show letter preview once file is uploaded

This shows the sanitised letter preview if the file had no validation
errors or the preview with the overlay if it failed validation.

app/s3_client/s3_letter_upload_client.py

@@ -1,3 +1,4 @@
+from boto3 import resource
 from flask import current_app
 from notifications_utils.s3 import s3upload as utils_s3upload
 
@@ -14,3 +15,13 @@ def upload_letter_to_s3(data, file_location, status):
         file_location=file_location,
         metadata={'status': status}
     )
+
+
+def get_letter_pdf_and_metadata(file_location):
+    s3 = resource('s3')
+    s3_object = s3.Object(current_app.config['TRANSIENT_UPLOADED_LETTERS'], file_location).get()
+
+    pdf = s3_object['Body'].read()
+    metadata = s3_object['Metadata']
+
+    return pdf, metadata