[TSVB] Rainbow palette should use colors from the color service

[wylieconlon]

When using the TSVB rainbow palette, the colors are assigned by position using a custom list of ~20 colors that isn't used anywhere else in Kibana. Instead, the color mapping service used by Visualize should be used in this case.

The goal of this change is to create a consistent dashboard experience, creating consistency. You can see the inconsistency when comparing two identical visualizations side-by-side, where one is using Visualize and the other is using TSVB:

Unlike making a change to the default palette in visualize, TSVB doesn't currently support overriding any part of the rainbow palette. This should make it a minor change for users.

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[wylieconlon]

Looks like this is not as easy technically as I thought: TSVB is handling colors on the server-side, while the color service used in dashboards is done client-side.

[wylieconlon]

An example of what we are targeting is below: