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.

tests/app/s3_client/test_s3_csv_client.py

@@ -0,0 +1,21 @@
+from unittest.mock import Mock
+
+from app.s3_client.s3_csv_client import set_metadata_on_csv_upload
+
+
+def test_sets_metadata(client, mocker):
+    mocked_s3_object = Mock()
+    mocked_get_s3_object = mocker.patch(
+        'app.s3_client.s3_csv_client.get_csv_upload',
+        return_value=mocked_s3_object,
+    )
+
+    set_metadata_on_csv_upload('1234', '5678', foo='bar', baz=True)
+
+    mocked_get_s3_object.assert_called_once_with('1234', '5678')
+    mocked_s3_object.copy_from.assert_called_once_with(
+        CopySource='test-notifications-csv-upload/service-1234-notify/5678.csv',
+        Metadata={'baz': 'True', 'foo': 'bar'},
+        MetadataDirective='REPLACE',
+        ServerSideEncryption='AES256',
+    )