search: Extract people.huddle_concat().

This is a pure code extraction.  The current
code is buggy with respect to user_ids with
different lengths of digits, i.e. it does
a naive lexical sort instead of a numerical
sort.  We'll fix that in the next commit.

frontend_tests/node_tests/people.js

@@ -538,6 +538,33 @@ run_test('multi_user_methods', () => {
 
 initialize();
 
+run_test('concat_huddle', () => {
+    /*
+        We assume that user_ids passed in
+        to concat_huddle have already been
+        validated, so we don't need actual
+        people for these tests to pass.
+
+        That may change in the future.
+    */
+
+    const user_ids = [303, 301, 302];
+
+    assert.equal(
+        people.concat_huddle(user_ids, 304),
+        '301,302,303,304'
+    );
+
+    // This is broken, but reflects the current
+    // state of the code.  We need a numerical sort.
+    assert.equal(
+        people.concat_huddle(user_ids, 99),
+        '301,302,303,99'
+    );
+});
+
+initialize();
+
 run_test('message_methods', () => {
     people.add(charles);
     people.add(maria);

static/js/people.js

@@ -352,6 +352,17 @@ function sorted_other_user_ids(user_ids) {
     return user_ids;
 }
 
+exports.concat_huddle = function (user_ids, user_id) {
+    /*
+        We assume user_ids and user_id have already
+        been validated by the caller.
+
+        The only logic we're encapsulating here is
+        how to encode huddles.
+    */
+    return user_ids.concat(user_id).sort().join(',');
+};
+
 exports.pm_lookup_key = function (user_ids_string) {
     /*
         The server will sometimes include our own user id

static/js/search_suggestion.js

@@ -60,8 +60,8 @@ function compare_by_huddle(huddle) {
     }
 
     return function (person1, person2) {
-        const huddle1 = huddle.concat(person1.user_id).sort().join(',');
-        const huddle2 = huddle.concat(person2.user_id).sort().join(',');
+        const huddle1 = people.concat_huddle(huddle, person1.user_id);
+        const huddle2 = people.concat_huddle(huddle, person2.user_id);
 
         // If not in the dict, assign an arbitrarily high index
         const score1 = huddle_dict.get(huddle1) || huddles.length + 1;