responsive Gantt-chart (#1724)

* default values for height and width of Gantt-chart are replaced with None
the created figure is now responsive
* default height is set to 600

Author: cs48a

Diff for: packages/python/plotly/plotly/figure_factory/_gantt.py

@@ -811,7 +811,7 @@ def create_gantt(
     showgrid_x=False,
     showgrid_y=False,
     height=600,
-    width=900,
+    width=None,
     tasks=None,
     task_names=None,
     data=None,