Store PublishedMetadata files in artifact storage
This patch changes PublishedMetadata into Content. This requires migrating the database.
The migration that comes with this change will only work on an empty database. It will
fail to migrate any existing PublishedMetadata. This is a backwards incompatible change.
The PublishedArtifact now has a classmethod called 'create_from_file'. This method creates
PublishedMetadata along with an Artifact, ContentArtifact, and PublishedArtifact.
Required PR: https://github.com/pulp/pulp_file/pull/278/