Change show_link default to False

In 1.43.0 Plotly.js will change its default behavior to hide the "Edit in Chart Studio" button by default.  Along with this, plotly.py should turn off the "Export to plotly.py" link by default.  With these changes, the online Chart Studio integration features will be opt-in when using plotly.py in "offline mode" (i.e. when using `plotly.offline.plot/iplot` or `plotly.graph_objs.FigureWidget`)