Detect whether the extension user is under censorship (therefore the extension is useless)

Currently we only say that in the extension's detailed description in stores. But some people who have installed it may have no idea that Tor is blocked where they live. If they are, there's no indication that it's not working (at least when the broker (snowflake-broker.freehaven.net) is reachable). Drawbacks:

• (I think) (when the broker is reachable) it still attempts to serve clients, but it fails, which is bad for clients as they waste time attempting to connect using this user's Snowflake extension.
• Extension user frustration as they get no feedback - the client count is always 0.

Related: legacy/trac#31391 (moved)