DashR docs sanity testing & Percy snapshots

[chriddyp]

It would be great if we had some sanity tests for Dash for R and some Percy tests too.

[Marc-Andre-Rivet]

Discussing this with @rpkyle, it feels like we should be able to leverage the existing DashPy docs code to navigate/discover all links to be tested. Assigning a relatively large size to account for R unknowns, possible problems and slow testing cycle for docs testing.

https://github.com/plotly/dash-docs/tree/master/tests/integration and https://github.com/plotly/dash-docs/tree/master/tests/unit