feat(base-app): Add base dash app file

This commit adds a file that contains a standard 'Hello World'
example of a plotly dash app.

app/app.py

@@ -0,0 +1,8 @@
+from dash import Dash, html
+
+app = Dash()
+
+app.layout = [html.Div(children="Hello World")]
+
+if __name__ == "__main__":
+    app.run(debug=True)