Find a way to get a "root url" in Jupyter

For the moment there is no nice way to make cross-apps link.

@oschuett pointed out that appmode injects a variable called `jupyter_notebook_url` that contains a notebook's url, something like:
https://server.com/user/user_id/notebooks/apps/some_folder/some_notebook.ipynb

However, there is no variable that points to Jupyter's root folder: https://server.com/user/user_id/notebooks/

Again, a suggestion from @oschuett was to use javascript for this purpose:
```
%%javascript
alert(Jupyter.notebook.base_url);
```
Unless there is a better way to do it I will stick to this solution.

@ltalirz @giovannipizzi @CasperWA any ideas?