isolate: configure port isolation from the web interface

A matrix of the ports, one row per ingress port, showing which ports a
frame entering there may leave through. The port number of a row turns
red when the port is not fully open, and the cell for the port itself is
marked since it is always allowed. Apply sends one "isolate" command per
changed row, and Allow all ticks the whole matrix.

The switch side is a new /isolation.json, which reports each port's
allowance mask read back from the hardware. The page is an Isolation
section of the single-page UI.

httpd/page_impl.h

@@ -18,6 +18,7 @@ void send_l2(uint16_t idx);
 void l2_delete(uint16_t idx);
 void send_mirror(void);
 void send_mtu(void);
+void send_isolation(void);
 void send_config(void);
 void send_cmd_log(void);
 void send_lag(void);