[change] Refactor ReceiveUrlAdmin to inherit CopyableFieldAdmin

[nemesifier]

It would be good to change ReceiveUrlAdmin to be based on the logic of CopyableFieldAdmin to reduce repetition of JS and python logic.

[pandafy]

Perform cleanup in openwisp-network-topology when this issue is fixed

https://github.com/openwisp/openwisp-network-topology/blob/5ed611adb00be5aae63b4a34da82da1202ffa16d/openwisp_network_topology/admin.py#L107-L113