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?

Signed-off-by: Jean-Christophe Jaskula <jean.christophe.jaskula+github@gmail.com>