Remove s3 dependency

Currently the 404 message depends on media residing in Joindiaspora's Amazon S3 account
Since Amazon is an unneded external dependency, we are leaking information by keeping the media there.

This PR changes the 404 message to use local media, and makes a copy from the Amazon S3 files.
This PR assumes that copyright on these files is with Joindiaspora, and that Joindiaspora agrees on including these files in the diaspora* distribution.