Updated App.js from latest version of AdminLTE - 

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #2275
| License       | MIT

This PR adds missing slimscroll for left menu.
These changes are copied from origin [app.js](https://github.com/almasaeed2010/AdminLTE/blob/master/js/AdminLTE/app.js) in AdminLTE.