feat(ui): hide "renew dhcp lease" button

[mattdibi]

This PR hides the "Renew DHCP lease" button from Kura web UI.

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When we switched from the old Kura networking to NetworkManager we lost the ability to renew the DHCP lease. We investigated the possibility to add this functionality back in. The results are the following:

1. There seems to be no "native" way to explicitly handle this in NetworkManager (for now) see Issue #1662.
2. There are alternative methods but either they are not really a "renewal" of the DHCP lease (i.e. result in lost connection) or they are quite invasive (see here) and rely on specific configurations (i.e. dhclient as dhcp client which might not be always true).
3. The real "DHCP lease renewal" should not lose the connection.

Given the above and the fact that the "Renew DHCP lease" functionality is not a hard requirement for our project, in agreement with @MMaiero, we decided to leave the feature out for now. When issue #1662 will be resolved we'll enable the feature back in.

To avoid confusion for our users we decided to hide the "Renew DHCP lease" button from Kura web UI, since it is currently non-functional. This is what this PR achieves.

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Results: