typeahead: Add pm_conversations module.

In pm_conversations.js, added function to make a user a PM partner and
another function to check if a user is a PM partner. A PM partner is
someone with whom the user has been in a PM with.

.eslintrc.json

@@ -136,6 +136,7 @@
         "current_msg_list": true,
         "home_msg_list": false,
         "pm_list": false,
+        "pm_conversations": false,
         "recent_senders": false,
         "unread_ui": false,
         "unread_ops": false,

frontend_tests/node_tests/message_store.js

@@ -1,6 +1,7 @@
 add_dependencies({
     people: 'js/people.js',
     util: 'js/util.js',
+    pm_conversations: 'js/pm_conversations.js',
 });
 
 var noop = function () {};

frontend_tests/node_tests/pm_conversations.js

@@ -0,0 +1,15 @@
+var assert = require('assert');
+var pmc = require('js/pm_conversations.js');
+
+(function test_partners() {
+    var user1_id = 1;
+    var user2_id = 2;
+    var user3_id = 3;
+
+    pmc.set_partner(user1_id);
+    pmc.set_partner(user3_id);
+
+    assert.equal(pmc.is_partner(user1_id), true);
+    assert.equal(pmc.is_partner(user2_id), false);
+    assert.equal(pmc.is_partner(user3_id), true);
+}());

static/js/message_store.js

@@ -49,6 +49,10 @@ exports.process_message_for_recent_private_messages = function (message) {
         return;
     }
 
+    _.each(user_ids, function (user_id) {
+        pm_conversations.set_partner(user_id);
+    });
+
     var user_ids_string = user_ids.join(',');
 
     if (!user_ids_string) {

static/js/pm_conversations.js

@@ -0,0 +1,20 @@
+var pm_conversations = (function () {
+
+var exports = {};
+
+var partners = new Dict();
+
+exports.set_partner = function (user_id) {
+    partners.set(user_id, true);
+};
+
+exports.is_partner = function (user_id) {
+    return partners.get(user_id) || false;
+};
+
+return exports;
+}());
+
+if (typeof module !== 'undefined') {
+    module.exports = pm_conversations;
+}

zproject/settings.py

@@ -875,6 +875,7 @@ JS_SPECS = {
             'js/unread.js',
             'js/topic_list.js',
             'js/pm_list.js',
+            'js/pm_conversations.js',
             'js/recent_senders.js',
             'js/stream_sort.js',
             'js/topic_generator.js',