setting(stream): Support user group pills when typeahead is unused.

This extra commit adds support for creating user group pills
in cases that do not use typeahead like, pasting the group
name, copying it from the user group pill.

This completes the remaining work required to support
addition of all members of a user groups to stream.

Fixes #15186.

frontend_tests/node_tests/user_group_pill.js

@@ -0,0 +1,54 @@
+"use strict";
+
+const {strict: assert} = require("assert");
+
+const {zrequire} = require("../zjsunit/namespace");
+const {run_test} = require("../zjsunit/test");
+
+const user_groups = zrequire("user_groups");
+const user_group_pill = zrequire("user_group_pill");
+
+const admins = {
+    name: "Admins",
+    description: "foo",
+    id: 1,
+    members: [10, 20],
+};
+const testers = {
+    name: "Testers",
+    description: "bar",
+    id: 2,
+    members: [20, 30, 40],
+};
+
+const admins_pill = {
+    id: admins.id,
+    group_name: admins.name,
+    display_value: admins.name + ": " + admins.members.length + " users",
+};
+const testers_pill = {
+    id: testers.id,
+    group_name: testers.name,
+    display_value: testers.name + ": " + testers.members.length + " users",
+};
+
+const groups = [admins, testers];
+for (const group of groups) {
+    user_groups.add(group);
+}
+
+run_test("create_item", () => {
+    function test_create_item(group_name, current_items, expected_item) {
+        const item = user_group_pill.create_item_from_group_name(group_name, current_items);
+        assert.deepEqual(item, expected_item);
+    }
+
+    test_create_item(" admins ", [], admins_pill);
+    test_create_item("admins", [testers_pill], admins_pill);
+    test_create_item("admins", [admins_pill], undefined);
+    test_create_item("unknown", [], undefined);
+});
+
+run_test("get_stream_id", () => {
+    assert.equal(user_group_pill.get_group_name_from_item(admins_pill), admins.name);
+});