minor: Avoid dict lookup for color.

The only thing get_color() does is look
up a sub:

    exports.get_color = function (stream_name) {
        const sub = exports.get_sub(stream_name);
        if (sub === undefined) {
            return stream_color.default_color;
        }
        return sub.color;
    };

So if we have a sub already, there's no point
calling the helper.

Obviously, this isn't a huge deal, but it happens
N times during page load.

static/js/stream_list.js

@@ -216,7 +216,7 @@ function build_stream_sidebar_li(sub) {
         is_muted: stream_data.is_muted(sub.stream_id) === true,
         invite_only: sub.invite_only,
         is_web_public: sub.is_web_public,
-        color: stream_data.get_color(name),
+        color: sub.color,
         pin_to_top: sub.pin_to_top,
     };
     args.dark_background = stream_color.get_color_class(args.color);