Added condition requiring content to print the "Download XML" link for descMetadata, rightsMetadata and defaultRights datastreams (fixes #806)
Tab, TabAction, and Tabs are now independent classes.
Moved tab methods from TabbedViewBehavior to the appropriate concern or controller for each method.
Merge pull request #825 from duke-libraries/806-download-xml

Added condition requiring content to print the "Download XML" link for d...