Add opt-in "online" + latency check on the client side

[laurentsenta]

Support to #442, we want to get rid of the "red message of death" that shows on a user's browser when the page starts calling gateways that might have been flagged with malicious content.

We'll generate most of the dashboard on the server side, but the liveness of a gateway and the latency with the client has to be computed locally. We'll make this feature opt-in.

In the dashboard, we'll

• add a button to let the user run a "ping" one or all gateways (opt-in, client-side)
• render the liveness & delta time icon once they are computed