minor: Avoid dict lookup for color.
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.

andersk/zulip,kou/zulip,rht/zulip,hackerkid/zulip,showell/zulip,showell/zulip,zulip/zulip,andersk/zulip,brainwane/zulip,kou/zulip,zulip/zulip,punchagan/zulip,timabbott/zulip,synicalsyntax/zulip,showell/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,synicalsyntax/zulip,kou/zulip,brainwane/zulip,hackerkid/zulip,brainwane/zulip,zulip/zulip,zulip/zulip,kou/zulip,zulip/zulip,showell/zulip,synicalsyntax/zulip,synicalsyntax/zulip,timabbott/zulip,shubhamdhama/zulip,punchagan/zulip,synicalsyntax/zulip,punchagan/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,brainwane/zulip,zulip/zulip,shubhamdhama/zulip,brainwane/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,shubhamdhama/zulip,punchagan/zulip,punchagan/zulip,hackerkid/zulip,timabbott/zulip,rht/zulip,andersk/zulip,synicalsyntax/zulip,brainwane/zulip,rht/zulip,shubhamdhama/zulip,eeshangarg/zulip,synicalsyntax/zulip,timabbott/zulip,rht/zulip,shubhamdhama/zulip,shubhamdhama/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,showell/zulip,shubhamdhama/zulip,timabbott/zulip,showell/zulip,showell/zulip,timabbott/zulip,kou/zulip,rht/zulip,timabbott/zulip,brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,rht/zulip