Use user's current full name when rendering messages.

The field message.sender_full_name can be out of date, so
we recompute is using data from people.js in
add_message_metadata().

frontend_tests/node_tests/message_store.js

@@ -50,6 +50,7 @@ var message_store = require('js/message_store.js');
 (function test_add_message_metadata() {
     var message = {
         sender_email: 'me@example.com',
+        sender_id: me.user_id,
         type: 'private',
         display_recipient: [me, bob, cindy],
         flags: ['has_alert_word'],

static/js/message_store.js

@@ -111,6 +111,11 @@ function add_message_metadata(message) {
 
     people.extract_people_from_message(message);
 
+    var sender = people.get_person_from_user_id(message.sender_id);
+    if (sender) {
+        message.sender_full_name = sender.full_name;
+    }
+
     switch (message.type) {
     case 'stream':
         message.is_stream = true;

static/js/people.js

@@ -28,6 +28,10 @@ exports.init = function () {
 exports.init();
 
 exports.get_person_from_user_id = function (user_id) {
+    if (!people_by_user_id_dict.has(user_id)) {
+        blueslip.error('Unknown user_id in get_person_from_user_id: ' + user_id);
+        return undefined;
+    }
     return people_by_user_id_dict.get(user_id);
 };