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

The colors for all the previews in the wizard, are found using getCurrentColors

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.