This PR makes some updates to colorscale handling to smooth over the distinction between specifying colorscales to plotly express functions and to graph objects.

1. Update the colorscale validator to accept lists of colors, and automatically construct a plotly.js colorscale as a list of pairs with uniformly spaced colors. Previously this was the only way that plotly express accepted custom color scales and it was not supported by graph object colorscales.
2. Update the colorscale validator to accept the name of colorscales from the sequential, diverging, and cyclical modules in the plotly.colors package.
3. Add definitions for the remaining plotly.js built-in color scales to plotly.colors. Now the colorscale is never left as a string, it is always coerced into a list-of-pairs color scale.
4. Update the plotly.express functions to use the colorscale validator, thus accepting all of the same colorscale forms as the graph objects.
5. Added the 'rebeccapurple' named css color.
6. Change default colorscale for the annotated heatmap figure factory to Plasma
7. Rename Plotly sequential colorscale to Plotly3 since this was only
   the theme default for version 3

@nicolaskruchten