stream edit: Extract stream_data.potential_subscribers().

We want to move more logic to stream_data to facilitate
testing.

Both before and after this commit, we essentially build a
new list of users for typeahead, but now the new list
excludes subscribed users.  We can do even better than
this in a follow-up commit.

static/js/people.js

@@ -675,6 +675,16 @@ exports.filter_all_persons = function (pred) {
     return ret;
 };
 
+exports.filter_all_users = function (pred) {
+    const ret = [];
+    for (const person of active_user_dict.values()) {
+        if (pred(person)) {
+            ret.push(person);
+        }
+    }
+    return ret;
+};
+
 exports.get_realm_persons = function () {
     return Array.from(active_user_dict.values());
 };

static/js/stream_data.js

@@ -371,6 +371,39 @@ exports.update_subscribers_count = function (sub) {
     sub.subscriber_count = count;
 };
 
+exports.potential_subscribers = function (sub) {
+    /*
+        This is a list of unsubscribed users
+        for the current stream, who the current
+        user could potentially subscribe to the
+        stream.  This may include some bots.
+
+        We currently use it for typeahead in
+        stream_edit.js.
+
+        This may be a superset of the actual
+        subscribers that you can change in some cases
+        (like if you're a guest?); we should refine this
+        going forward, especially if we use it for something
+        other than typeahead.  (The guest use case
+        may be moot now for other reasons.)
+    */
+
+    function is_potential_subscriber(person) {
+        // Use verbose style to force better test
+        // coverage, plus we may add more conditions over
+        // time.
+        if (sub.subscribers.has(person.user_id)) {
+            return false;
+        }
+
+        return true;
+    }
+
+    return people.filter_all_users(is_potential_subscriber);
+
+};
+
 exports.update_stream_email_address = function (sub, email) {
     sub.email_address = email;
 };

static/js/stream_edit.js

@@ -216,7 +216,7 @@ function show_subscription_settings(sub_row) {
     }).init();
 
     sub_settings.find('input[name="principal"]').typeahead({
-        source: people.get_realm_persons, // This is a function.
+        source: () => stream_data.potential_subscribers(sub),
         items: 5,
         highlighter: function (item) {
             return typeahead_helper.render_person(item);
@@ -227,10 +227,8 @@ function show_subscription_settings(sub_row) {
                 return false;
             }
             // Case-insensitive.
-            const item_matches = item.email.toLowerCase().includes(query) ||
-                               item.full_name.toLowerCase().includes(query);
-            const is_subscribed = stream_data.is_user_subscribed(sub.name, item.user_id);
-            return item_matches && !is_subscribed;
+            return item.email.toLowerCase().includes(query) ||
+                   item.full_name.toLowerCase().includes(query);
         },
         sorter: function (matches) {
             const current_stream = compose_state.stream_name();