fix: the default title in the template is always Notebook

The property used as a fallback for the title of the template is
not initialized by Voila. Following nbconvert, in which this
property is set to the filename of the notebook [1], this commit
now does the same.

Related to: voila-dashboards/voila-vuetify#25

[1] https://github.com/jupyter/nbconvert/blob/6c6e5fa0c757169017fd325fad98efa8d4a368ea/nbconvert/exporters/exporter.py#L172

voila/handler.py

@@ -59,11 +59,17 @@ async def get(self, path=None):
             return
         self.cwd = os.path.dirname(notebook_path)
 
+        path, basename = os.path.split(notebook_path)
+        notebook_name = os.path.splitext(basename)[0]
+
         # render notebook to html
         resources = {
             'base_url': self.base_url,
             'nbextensions': nbextensions,
-            'theme': self.voila_configuration.theme
+            'theme': self.voila_configuration.theme,
+            'metadata': {
+                'name': notebook_name
+            }
         }
 
         # include potential extra resources