Move subnets in and out of the shared network using the shared network form

The shared network form implemented with #1370 (closed) allows for modifying the DHCP parameters and options, but does not allow for moving the subnets into the shared network and removing them from the shared network. The only way to do it is by navigating into the subnet form. This issue proposes that the subnets are listed as "chips" in the shared network form (similar to servers) and can be easily added or removed from the shared network.