Add FetchStatus class.

We will use this in subsequent commits to replace the
`load_more_enabled` flag.

static/js/fetch_status.js

@@ -0,0 +1,41 @@
+var FetchStatus = function () {
+
+    var self = {};
+
+    var loading_older = false;
+    var loading_newer = false;
+    var found_oldest = false;
+    var found_newest = false;
+
+    self.start_older_batch = function () {
+        loading_older = true;
+    };
+
+    self.finish_older_batch = function (opts) {
+        loading_older = false;
+        found_oldest = opts.found_oldest;
+    };
+
+    self.can_load_older_messages = function () {
+        return !loading_older && !found_oldest;
+    };
+
+    self.start_newer_batch = function () {
+        loading_newer = true;
+    };
+
+    self.finish_newer_batch = function (opts) {
+        loading_newer = false;
+        found_newest = opts.found_newest;
+    };
+
+    self.can_load_newer_messages = function () {
+        return !loading_newer && !found_newest;
+    };
+
+    return self;
+
+};
+if (typeof module !== 'undefined') {
+    module.exports = FetchStatus;
+}