This commit adds server-side functionality for the [jupyter-renderjs-extension](https://lab.nexedi.com/Kreisel/jupyter_renderjs_extension). This has no interaction/impact with the existing functionality of the erp5_data_notebook BT.

@klaus mentioned that it is not common practice to return objects of a class (defined in the script). However, for usuability reasons there is an argument to do it here:
Usually when using a jupyter extension, a module is imported an functionality is provided by that module. For instance:
```python
In [1]:
import mymodule as mm
mm.someFunction()
mm.someOtherFunction()
```

To emulate this behavior **and** to be consistent with the [ipython-version (Python 2 Kernel)](https://lab.nexedi.com/Kreisel/jupyter_renderjs_extension/blob/master/renderjs_ipyextension/renderjs_ipyextension/renderjs_extension.py) of this extension I therefore use an object of the class `RJSExtension` which is returned by this script. I am then able to use
```python
rjs = Base_loadRenderJSExtension()
rjs.someFunction()
rjs.someOtherFunction()
```

instead of something like

```python
Base_rjsExtensionSomeFunction()
Base_rjsExtensionSomeOtherFunction()
```

which is much less user-friendly. If there is a better alternative I am not aware of, please comment. Also if anything else is amiss.

/reviewed-on !238