Show an 'empty' tab if the user has no access to any browser plugins

Since we changed the media widget to not require the 'edit files' permission to use the widget, we've had a reoccurring problem that some users don't have access to any plugin tabs get really confused because the browser modal pops up but the user cannot really see anything nor dismiss it. We should add an 'Empty' tab that says "You do not have access to any media browser plugins. Contact your site administrator for help."