deps: Move xdate from static/third to npm

- Remove `xdate` from `static/third` and fetch it from `npm`.
- Bump up the `PROVISION_VERSION` to 4.3.

Part of #1709.

frontend_tests/node_tests/message_edit.js

@@ -2,7 +2,7 @@ set_global('document', null);
 set_global('page_params', {});
 
 add_dependencies({
-    XDate: 'third/xdate/xdate.dev.js',
+    XDate: 'node_modules/xdate/src/xdate.js',
 });
 
 var message_edit = require('js/message_edit.js');