Just use message.unread when rendering.

By the time we render messages, we will have set message.unread,
so we don't need to calculate it from flags.

We add a line to the local-echo path to make this explicit
in that code.

static/js/echo.js

@@ -90,6 +90,7 @@ function insert_local_message(message_request, local_id) {
     // Locally delivered messages cannot be unread (since we sent them), nor
     // can they alert the user.
     message.flags = ['read']; // we may add more flags later
+    message.unread = false;
 
     message.raw_content = message.content;