Stop persisting email branding logos if saving to db fails

We were previously persisting the logo for the email branding and
deleting the temp files that get created before trying to update the
database with the new email branding. This meant that if there was an
error when saving (e.g. the domain used was a duplicate) the final logo
was already in S3 and trying to go 'back' in the browser would give an
error since the temp files needed to display the create branding page
had already been deleted.

This changes the order we do things in, so that we try persisting the
email branding to the database first.

app/main/s3_client.py

@@ -31,7 +31,9 @@ def delete_s3_object(filename):
     get_s3_object(bucket_name, filename).delete()
 
 
-def rename_s3_object(old_name, new_name):
+def persist_logo(old_name, new_name):
+    if old_name == new_name:
+        return
     bucket_name = current_app.config['LOGO_UPLOAD_BUCKET_NAME']
     get_s3_object(bucket_name, new_name).copy_from(
         CopySource='{}/{}'.format(bucket_name, old_name))
@@ -110,17 +112,12 @@ def upload_logo(filename, filedata, region, user_id):
     return upload_file_name
 
 
-def persist_logo(filename, user_id):
+def permanent_logo_name(filename, user_id):
     if filename.startswith(TEMP_TAG.format(user_id=user_id)):
-        persisted_filename = get_temp_truncated_filename(
-            filename=filename, user_id=user_id)
+        return get_temp_truncated_filename(filename=filename, user_id=user_id)
     else:
         return filename
 
-    rename_s3_object(filename, persisted_filename)
-
-    return persisted_filename
-
 
 def delete_temp_files_created_by(user_id):
     for obj in get_s3_objects_filter_by_prefix(TEMP_TAG.format(user_id=user_id)):