Add the option to edit a page from the Userbar

[saevarom]

I have been experimenting with a headless setup and the dynamic Userbar. What I found lacking is the option to edit a page from the Userbar. This PR along with an edit to the feature in the headless branch of bakerydemo will accomplish that.

The changes needed in the backend to accomplish these changes are:

class UserbarView(TemplateView):
    template_name = Userbar.template_name
    http_method_names = ["get"]

    def dispatch(self, request, *args, **kwargs):
        response = super().dispatch(request, *args, **kwargs)
        client_url = headless_preview_settings.CLIENT_URLS["default"]
        response["Access-Control-Allow-Origin"] = client_url
        return response

    def get_context_data(self, **kwargs):
+        object = None
+        page_id = self.request.GET.get('page_id', None)
+        if page_id:
+            object = Page.objects.filter(id=page_id).first()

-        return Userbar(object=None, position="bottom-left").get_context_data(
+        return Userbar(object=object, position="bottom-left").get_context_data(
            super().get_context_data(request=self.request, **kwargs)
        )