minor server-sent events fixes (#12945)

* Send output on the server-sent events stream immediately so the client sees that it was successfully opened even if it doesn't have any messages.

Fix transparent SSE streaming for the public:local and hashtag:local stream types.

* Tell caches to never store server-sent events.

app/javascript/mastodon/stream.js

@@ -92,6 +92,7 @@ export default function getStream(streamingAPIBaseURL, accessToken, stream, { co
     return ws;
   }
 
+  stream = stream.replace(/:/g, '/');
   params.push(`access_token=${accessToken}`);
   const es = new EventSource(`${streamingAPIBaseURL}/api/v1/streaming/${stream}?${params.join('&')}`);