Synchronize Data Points Color the same across all widgets on dashboard

## What?

Currently, when you have many widgets on a dashboard looking at a data set, the same source can show up in different colors. This can be confusing when doing visual correlation of data, when looking at spikes etc.

In example below, on the pie chart, Orange and Blue are the large slices, but when looking at the line chart, green and blue are the corresponding lines. We need to have a way to get the colors locked in per source/data point.

## Why?
This will help the analyst quickly visualize the the data on the screen and make decisions knowing the colors are the same across all data points on the dashboards.

## Your Environment

* Graylog Version: 3.2.4+