remove messages from S3 when appropriate

[amyxzhang]

When a message held for moderation is approved, we delete it from our database. We should also remove it from the S3 bucket.

For rejected messages:

• if the owner has "store rejected messages on site" turned on, we give them the option to later permanently delete the message from the site. when they do that, we should also delete it from S3.
• if the owner doesn't want us to store rejected messages, we should delete it from S3 upon rejection.

we need to add a function to delete from S3 (we already have a function to upload, which should be somewhat similar) and then call it in these places.