Make it more obvious when you can safely delete an assetstore.

Disable the delete button for assetstores that have files in them.  When delete is selected, reassure the user that no data will be lost.  We still throw an error if a file is added to the assetstore before the user has a chance to approve the deletion.
This is a helpful message, I think we should put a bootstrap tooltip on it. I usually just do it in the `render` method, right after the template gets rendered:

    this.$('.g-assetstore-button-container[title]').tooltip();