erp5_data_notebook: Add script for renderjs-extension

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

bt5/erp5_data_notebook/SkinTemplateItem/portal_skins/erp5_data_notebook/Base_loadRenderJSExtension.py

+import json
+
+# This script provides the backend-functionality of the Juypter Notebook RenderJS Extension.
+# Due to the internal protocol of the ERP5Kernel and the ERP5-Jupyter-Backend messages between
+# Javascript in the client and the ERP5Kernel are exchange via objections containing
+# Javascript code as _repr_html_.
+
+# Here a schematic overview of the messaging:
+# 1. Extension-Function is called in the notebook (e.g. loadGadget("gadget", "https://someurl.com/gadget"))
+# 2. Code is processed by the ERP5Kernel (client)
+# 3. Code is sent to the ERP5-Backend
+# 4. The required logic is handled by this script in the backend
+# 5. This script returns an object with _repr_html_ containing the JS-response for the client
+# 6. The ERP5-Backend sends a response to the ERP5Kernel (client)
+# 7. The ERP5Kernel interprets the object with _repr_html_ as text/html message and injects it into the notebook
+# 8. Now the Javascript code is executed as part of the (client) extension (e.g. a renderJS-gadget is loaded into the page)
+
+class RJSExtension:
+
+  def __init__(self):
+    pass
+
+
+  # Create the original load_gadget with modified rsvp, renderjs
+  # Because jupyter notebook has already loaded when this can be called
+  # a manual initialization of the whole renderJS setup is required
+  #
+  # First the libs rsvp, renderjs-gadget-global and renderjs (patched)
+  # are injected into the page. The patch on renderjs itself is to enable
+  # the following manual bootstrap
+
+  # After the scripts are present, a div is appended containing the
+  # loading_gadget.
+
+  # After everything is inplace, rJS.manualBootstrap initializes the
+  # loading_gadget in exactly the same way as when rJS is normally initialized
+  # (on-load)
+  def initRenderJS(self):
+    script = '''
+    <script>
+    var loadingDiv = document.querySelector(".loading_gadget");
+    if(loadingDiv == null) {
+      console.log("~~ Initializing RenderJS!");
+      $.getScript("/nbextensions/renderjs_nbextension/rsvp-2.0.4.js", function() {
+        console.log("~~ loading_gadget: rsvp.js loaded");
+        $.getScript("/nbextensions/renderjs_nbextension/rjs_gadget_global_js.js", function() {
+          console.log("~~ loading_gadget: renderjs-gadget-global.js loaded");
+          $.getScript("/nbextensions/renderjs_nbextension/renderjs-latest.js", function() {
+            console.log("~~ loading_gadget: renderjs.js loaded");
+            $("#notebook-container").append('<div data-gadget-url="/nbextensions/renderjs_nbextension/loading_gadget.html" data-gadget-scope="public"></div>');
+            rJS.manualBootstrap();
+          });
+        });
+      });
+    } else {
+      console.log("~~ Renderjs seems to be initialized already!");
+    }
+    </script>'''
+    return RJSHtmlMessage(script)
+
+
+  # Load a gadget given a unique ref and URL to the HTML file of the gadget
+  # -> Fires an event which loading_gadget listens on and passes on the URL
+  def loadGadget(self, ref, gadgetUrl):
+    script = '''
+    <script>
+    var load_event = new CustomEvent("load_gadget",
+    { "detail": { "url": "''' + gadgetUrl + '", "gadgetId": "' + ref + '''" }});
+
+    var loadingDiv = document.querySelector(".loading_gadget");
+    if(loadingDiv != null) {
+      loadingDiv.dispatchEvent(load_event);
+    } else {
+      console.log("~~ load: RenderJS init required first!");
+    }
+    </script>
+    '''
+    return RJSHtmlMessage(script)
+
+
+  # Fires an event with
+  #    * the ref of the gadget
+  #    * the name of the declared_method
+  #    * the arguments to be passed to the declared_method
+  # The arguments are packed into a json string and passed to js as such
+  def callDeclaredMethod(self, ref, method_name, *args):
+    j_str = json.dumps(args)
+    script = '''
+    <script>
+    var call_event = new CustomEvent("call_gadget",
+    { "detail": {
+    "gadgetId": "''' + ref + '''",
+    "methodName": "''' + method_name + '''",
+    "methodArgs": ''' + "'" + j_str + "'" + '''
+    }});
+    var loadingDiv = document.querySelector(".loading_gadget");
+    if(loadingDiv != null) {
+    loadingDiv.dispatchEvent(call_event);
+    } else {
+    console.log("~~ call: RenderJS init required first!");
+    }
+    </script>
+    '''
+    return RJSHtmlMessage(script)
+
+
+  # Fires an event to the destroy this gadget
+  # Only thing passed is the ref of the gadget
+  def destroyGadget(self, ref):
+    script = '''
+    <script>
+    var destroy_event = new CustomEvent("destroy_gadget",
+    { "detail": { "gadgetId": "''' + ref + '''" }});
+    var loadingDiv = document.querySelector(".loading_gadget");
+    if(loadingDiv != null) {
+      loadingDiv.dispatchEvent(destroy_event);
+    } else {
+      console.log("~~ destroy: RenderJS init required first!");
+    }
+    </script>
+    '''
+    return RJSHtmlMessage(script)
+
+
+class RJSHtmlMessage:
+  '''
+    Represents a HTML-injection into the frontend. Returning such an object from the ERP5
+    backend is sufficient, as the _repr_html_ will be called internally.
+  '''
+  def __init__(self, html):
+    self.html = html
+  def _repr_html_(self):
+    return self.html
+
+obj = RJSExtension()
+return obj

bt5/erp5_data_notebook/SkinTemplateItem/portal_skins/erp5_data_notebook/Base_loadRenderJSExtension.xml

+<?xml version="1.0"?>
+<ZopeData>
+  <record id="1" aka="AAAAAAAAAAE=">
+    <pickle>
+      <global name="PythonScript" module="Products.PythonScripts.PythonScript"/>
+    </pickle>
+    <pickle>
+      <dictionary>
+        <item>
+            <key> <string>Script_magic</string> </key>
+            <value> <int>3</int> </value>
+        </item>
+        <item>
+            <key> <string>_bind_names</string> </key>
+            <value>
+              <object>
+                <klass>
+                  <global name="NameAssignments" module="Shared.DC.Scripts.Bindings"/>
+                </klass>
+                <tuple/>
+                <state>
+                  <dictionary>
+                    <item>
+                        <key> <string>_asgns</string> </key>
+                        <value>
+                          <dictionary>
+                            <item>
+                                <key> <string>name_container</string> </key>
+                                <value> <string>container</string> </value>
+                            </item>
+                            <item>
+                                <key> <string>name_context</string> </key>
+                                <value> <string>context</string> </value>
+                            </item>
+                            <item>
+                                <key> <string>name_m_self</string> </key>
+                                <value> <string>script</string> </value>
+                            </item>
+                            <item>
+                                <key> <string>name_subpath</string> </key>
+                                <value> <string>traverse_subpath</string> </value>
+                            </item>
+                          </dictionary>
+                        </value>
+                    </item>
+                  </dictionary>
+                </state>
+              </object>
+            </value>
+        </item>
+        <item>
+            <key> <string>_params</string> </key>
+            <value> <string></string> </value>
+        </item>
+        <item>
+            <key> <string>id</string> </key>
+            <value> <string>Base_loadRenderJSExtension</string> </value>
+        </item>
+      </dictionary>
+    </pickle>
+  </record>
+</ZopeData>