mgr/dashboard: fix rbd image 'purge trash' button & modal text
Pull request ID:
Block > Images > 'Trash' tab:
- The 'Purge Trash' button is enabled even when there are no images to purge.
Expected behavior: like linux file managers: "Purge Trash" greyed out (disabled) if there is nothing to remove.
- 'Purge Trash' modal text: "To purge, select one or All images and click..."
Expected behavior: it should say 'All pools' instead of 'All images'.