message_fetch: Use found_oldest flag from server.

We now use the found_oldest flag from the server to determine
whether we should load older messages.

This requires us to change a few places in the code where we
pass the whole payload around now instead of just the messages.

Actually, many of the "cont" callbacks don't even look at the
data, so this change wasn't as invasive as I might have
predicted.

static/js/message_fetch.js

@@ -14,7 +14,9 @@ var consts = {
     catch_up_batch_size: 1000,
 };
 
-function process_result(messages, opts) {
+function process_result(data, opts) {
+    var messages = data.messages;
+
     $('#connection-error').removeClass("show");
 
     if ((messages.length === 0) && (current_msg_list === message_list.narrowed) &&
@@ -44,7 +46,7 @@ function process_result(messages, opts) {
     pm_list.update_private_messages();
 
     if (opts.cont !== undefined) {
-        opts.cont(messages);
+        opts.cont(data);
     }
 }
 
@@ -63,7 +65,7 @@ function get_messages_success(data, opts) {
         return;
     }
 
-    process_result(data.messages, opts);
+    process_result(data, opts);
     resize.resize_bottom_whitespace();
 }
 
@@ -108,7 +110,10 @@ exports.load_messages = function (opts) {
                 // retry or display a connection error.
                 //
                 // FIXME: Warn the user when this has happened?
-                process_result([], opts);
+                var data = {
+                    messages: [],
+                };
+                process_result(data, opts);
                 return;
             }
 
@@ -164,11 +169,10 @@ exports.maybe_load_older_messages = function (opts) {
         num_before: batch_size,
         num_after: 0,
         msg_list: msg_list,
-        cont: function (messages) {
+        cont: function (data) {
             opts.hide_loading();
-            var found_oldest = messages.length < batch_size;
             msg_list.fetch_status.finish_older_batch({
-                found_oldest: found_oldest,
+                found_oldest: data.found_oldest,
             });
         },
     });
@@ -176,7 +180,8 @@ exports.maybe_load_older_messages = function (opts) {
 
 exports.initialize = function () {
     // get the initial message list
-    function load_more(messages) {
+    function load_more(data) {
+        var messages = data.messages;
 
         // If we received the initially selected message, select it on the client side,
         // but not if the user has already selected another one during load.