Refactor to split s3_client.py into multiple files

Separated s3_client.py into 3 files - for logos, CSV files and the MOU.
This helps to keep things clearer now that we need to add lots more logo
functions for letters.

app/s3_client/s3_mou_client.py

@@ -0,0 +1,24 @@
+import botocore
+from flask import current_app
+
+from app.s3_client.s3_logo_client import get_s3_object
+
+
+def get_mou(organisation_is_crown):
+    bucket = current_app.config['MOU_BUCKET_NAME']
+    filename = 'crown.pdf' if organisation_is_crown else 'non-crown.pdf'
+    attachment_filename = 'GOV.UK Notify data sharing and financial agreement{}.pdf'.format(
+        '' if organisation_is_crown else ' (non-crown)'
+    )
+    try:
+        key = get_s3_object(bucket, filename)
+        return {
+            'filename_or_fp': key.get()['Body'],
+            'attachment_filename': attachment_filename,
+            'as_attachment': True,
+        }
+    except botocore.exceptions.ClientError as exception:
+        current_app.logger.error("Unable to download s3 file {}/{}".format(
+            bucket, filename
+        ))
+        raise exception