streams: Add LazySet for subscribers.

This defers O(N*S) operations, where

    N = number of streams
    S = number of subscribers per stream

In many cases we never do an O(N) operation on
a stream.  Exceptions include:

    - checking stream links from the compose box
    - editing a stream
    - adding members to a newly added stream

An operation that used to be O(N)--computing
the number of subscribers--is now O(1), and we
don't even pay O(N) on a one-time basis to
compute it (not counting the cost to build the
array from JSON, but we have to do that).

static/js/stream_edit.js

@@ -36,12 +36,9 @@ exports.is_sub_settings_active = function (sub) {
 };
 
 exports.get_email_of_subscribers = function (subscribers) {
-    const emails = [];
-    subscribers.each(function (o, i) {
-        const email = people.get_person_from_user_id(i).email;
-        emails.push(email);
+    return subscribers.map(function (user_id) {
+        return people.get_person_from_user_id(user_id).email;
     });
-    return emails;
 };
 
 function clear_edit_panel() {