Info stream (#336)

* add /stream/info as a cheap way to subscribe to comment updates #253

* add check for lastTS change to allow proper info streams in no-cache mode

* check write error in info stream and terminate

* flaky info stream test

* add stream info to readme

* separate timeout middleware foe each route's group

* debug info on stream close

* fix test for streams

* stream timeout on inactivity only

* throttle streams to 500

* restore common throttle

backend/app/cmd/server.go

@@ -304,6 +304,8 @@ func (s *ServerCommand) newServerApp() (*serverApp, error) {
 		SSLConfig:        sslConfig,
 		UpdateLimiter:    s.UpdateLimit,
 		ImageService:     imageService,
+		StreamTimeOut:    time.Minute * 15,
+		StreamRefresh:    time.Second,
 	}
 
 	srv.ScoreThresholds.Low, srv.ScoreThresholds.Critical = s.LowScore, s.CriticalScore