Add "New Circuit" to right-click context menu

See this suggestion from a forum user.

The natural position would seem to be below "Reload" in the context menu. We should probably use the existing string in title case (i.e. "New Tor Circuit For This Site"), however that's very wordy and I think "...For This Site" can be inferred based on the context, so I'd be in favor of slimming it down to "New Circuit" here.