be more explicit about the levels of permission in s3

[RZachLamberty]

even if we are attempting to breeze through s3 lectures on account of low interest, I think it is worth our time to slow down and present the permissions layers more concretely.

in particular, we should stress:

• granting co-ownership or public access to buckets during bucket creation is a list permission
• granting co-ownership or public access to a file during upload or from the console post-upload is a read permission
• granting permission to anything more specific than a single file requires an update of the bucket policy