Versions for assets in the catalog

The Community App Catalog does not have a concept of versions for individual assets. If a contributor wants to update an existing asset they can, but the only evidence that something has changed is the git log. Additionally contributors can choose to manually version their contributions, but trying to manage versions manually is problematic. Ideally the App Catalog will maintain information on previous versions, allow users to choose an older version, and alert users who are "watching" an asset when a new version becomes available.