Add REST API that allows the download manager to be the source of truth for all downloads

In order for us to just load all the downloads for the site from the download manager, we need to be able to query a few things. On a per product basis:

• what downloads are currently available
• which artifact (e.g. installer, zip) should be the one we promote (e.g. via a Download button for a product - see right rail on http://www.jboss.org/products/fuse/overview/)

For each product release we need:

• Version name
• Release Date (the date the product was released, should default to the date the entry is added to the download manager)
• Description (per file)
• Available downloads
• Download type (e.g. zip, installer, release_notes, docker image, source, md5, sha1) (default according to file extension, but allow the user to select others from a list)
• download group (optional) _(this allows to provide some visual indicator that this download is part of a group e.g. Hot Rod client for .NET, C++, Java etc.)
• whether the download is n alpha, beta or a GA (dropdown selected by the user)