[t95] object prefix for s3 storage

1. Changed SHARE_URL_PREFIX to PUBLISH_URL_PREFIX.
2. Added S3_OBJECT_PREFIX to .env files.
3. Updated README.
4. routes/store.js uses S3_OBJECT_PREFIX to prepend generated filenames.
5. Fixed publish url style.
Merge pull request #99 from secretrobotron/t95-object-prefix

[t95] object prefix for s3 storage