Disable subnet edit and delete buttons when subnet_cmds unavailable
As suggested in !674 (comment 418902), we should consider hiding or disabling the button for editing and deleting a subnet when there are no machines with the subnet_cmds hook loaded. We should probably also disable it when any of the daemons associated with the subnet does not have this hook loaded. There are possibly other cases and similar issues exist for host management.
The issue was noticed by @marcin during 1.15 sanity checks: #1296 (comment 434197)
An attempt to delete a subnet for which the servers have no subnet_cmds hook library loaded, disables the delete button and hangs for a while. Then, it displays a message in the top right corner saying that the subnet4-del
command is not supported.