Create AppCell.php

While working on adding Cell support to [TwigView](https://github.com/WyriHaximus/TwigView) I've noticed that not only in `AppController` the `viewClass` has to be set the to `TwigView` but also in the cells. Currently There is no `AppCell` to make this happen globally. This PR changes that. I've also included a link to the docs in case this PR gets merged.