Add arrow tab sections to home page.

This lets you use up and down arrows in certain sections on the site to
navigate lists. It also prevents potentially unwanted actions caused by
using up/down arrows in those sections.

Fixes: #1696.

static/js/hotkey.js

@@ -61,6 +61,25 @@ var hotkeys_shift_insensitive = {
     119: {name: 'query_streams', message_view_only: false} // 'w'
 };
 
+var tab_up_down = (function () {
+    var list = ["#group-pm-list", "#stream_filters", "#global_filters", "#user_presences"];
+
+    return function (e) {
+        var $target = $(e.target);
+        var flag = $target.closest(list.join(", "));
+
+        return {
+            flag: flag,
+            next: function () {
+                return $target.closest("li").next().find("a");
+            },
+            prev: function () {
+                return $target.closest("li").prev().find("a");
+            }
+        };
+    };
+}());
+
 function get_hotkey_from_event(e) {
 
     // We're in the middle of a combo; stop processing because
@@ -95,6 +114,17 @@ function process_hotkey(e) {
         return false;
     }
 
+    var tab_list = tab_up_down(e);
+    if (tab_list.flag) {
+        if (hotkey.name === "up_arrow") {
+            tab_list.prev().focus();
+            return true;
+        } else if (hotkey.name === "down_arrow") {
+            tab_list.next().focus();
+            return true;
+        }
+    }
+
     if (event_name === 'ignore') {
         return false;
     }