Bundle download - Store metadata and resolve hard coding path issues

Created by: deepakduggirala

Store name, md5 scheksum, size (currently - dataset.bundle_size) in the dataset_file table.

Storing bundle in dataset_file shouldn't impact the file listing APIs.

Bundle is assumed to be always a tar file and this is hard coded while constructing paths in UI, API, Workers code.

The download alias directory download/<alias> is symlinked to stage/<alias>/<dataset_name>. This is because of FACL constraints. This is an issue now because the bundle file is staged to stage/<alias>/<dataset_name>.tar and symlinked to download/<alias>/<dataset_name>.tar. This placement makes it look like <dataset_name>.tar is inside the dataset and as one of the files of the dataset for end users looking in download/<alias>

Show a button to download the bundle in the dataset page (it is being shown in project page now)

#37 (closed) seems easy enough to fix in this ticket.