Apply reversed colorscale setting and coloraxis in contour and histogramcontour legends

[archmoj]

Fixes #4354 and fixes #4355

• displays identical legends for identical graphs
• use coloraxis colorscale (if defined) to style legend
• use baseAttrs instead of plotAttrs in all trace/attributes.js files (refactor commit 0c00efc)

@alexcjohnson
@plotly/plotly_js
cc: @nicolaskruchten

[etpinard]

💃 nicely done @archmoj (once that flaky test passes)