Explorer import and display improvements

Some warnings at import show that:

• some useless listeners are performed (pictureViewsGenerationListener)
• details of import failure are not visible on server logs
• stats should be shown to control import/export of distributions, on all distributions home page (like on admin distribution home page)
• tabs should not be shown for empty content (maybe), for instance the "packages" tab should not be shown on partial distribs without packages gave up on this as it would be costly to check the number of each of the artifacts on persisted distributions (and overkill to setup persistence/cache for now)