lan: add ability to set static leases

this MAC address will get this IP, etc

Note that the backend part is supposed to be done

https://gitlab.labs.nic.cz/turris/foris-controller/foris-controller/blob/master/foris_controller_modules/lan/schema/lan.json#L185

Steps in this issue

• create endpoint (!259 (merged) @fhron)
• implement UI (This ticket)

Schema

"dhcp_client_set": {
            "type": "object",
            "properties": {
                "mac": {"type": "string", "format": "macaddress"},
                "ip": {
                    "oneOf": [
                        {"type": "string", "format": "ipv4"},
                        {"enum": ["ignore"]}
                    ]
                },
                "hostname": {"type": "string"}
            },
            "additionalProperties": false,
            "required": ["ip", "mac", "hostname"]
        }

enpoint

<API>/api/lan/set_client

Mock