Add keydown_util.js module.

This is a pretty thin abstraction to prevent having to put
magic numbers in code, doing the which/keyCode hack, and remembering
to all preventDefault.

Hopefully we'll expand it to handle things like shift/alt keys
for components that want their own keyboard handlers (vs. going
through hotkey.js).

static/js/keydown_util.js

@@ -0,0 +1,43 @@
+var keydown_util = (function () {
+
+var exports = {};
+
+/*
+    See hotkey.js for handlers that are more app-wide.
+*/
+
+var keys = {
+    37: 'left_arrow',
+    38: 'up_arrow',
+    39: 'right_arrow',
+    40: 'down_arrow',
+};
+
+exports.handle = function (opts) {
+    opts.elem.keydown(function (e) {
+        var key = e.which || e.keyCode;
+
+        var key_name = keys[key];
+
+        if (!key_name) {
+            return;
+        }
+
+        if (!opts.handlers[key_name]) {
+            return;
+        }
+
+        var handled = opts.handlers[key_name]();
+
+        if (handled) {
+            e.preventDefault();
+        }
+    });
+};
+
+return exports;
+}());
+
+if (typeof module !== 'undefined') {
+    module.exports = keydown_util;
+}