FIX: Wizard previews if color step is excluded (PR #9881)

The colors for all the previews in the wizard, are found using getCurrentColors https://github.com/discourse/discourse/blob/675e9b81c63494cdb62add81edbd86ac626714e5/app/assets/javascripts/wizard/lib/preview.js#L86

That function relies on the color step to be present to know the current color scheme colors. This is generally fine, but in this commit I added the ability for plugins to exclude plugin steps.

If a plugin excludes the color step, all of the previews will be blank, looking like this:

Screenshot from 2020-05-26 10-13-51

By adding the current_color_scheme to the wizard serializer, we have a fallback if the color step is not present.

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