Creating an entry point for notebook manager extensions

Following the comments about  #5989, I'm turning my multicolumn notebook manager hack into an extension. I have the feeling that the only way to load extensions as early as the notebook manager is to create a new entry point by:

- Firing app_initialized.DashboardApp event when loaded the notebook manager (in tree/js/main.js)
- Updating tree.html template to load nbextensions through custom.js and this newly created event.

Is this change necessary and acceptable to you?
`tree.html`  inherit `page.html` that already include `custom.js` you shouldn't need that : 
cf page source on master of `/tree/`:

```
    <script src="/static/tree/js/kernellist.js?v=09864573b75c7ed64f762c2982405faf" type="text/javascript" charset="utf-8"></script>
    <script src="/static/tree/js/clusterlist.js?v=f2204ec19237c846e50acee5063ec7af" type="text/javascript" charset="utf-8"></script>
    <script src="/static/tree/js/main.js?v=c21cd741b885921827e3cd05604356a2" type="text/javascript" charset="utf-8"></script>


<script src="/static/custom/custom.js?v=2d43ed8be103f0de4685c5b4f78abd44" type="text/javascript" charset="utf-8"></script>

</body>
```