Simplify theme and color scheme seeding (#10872)

Now that we have support for user-selectable color schemes, it makes sense
to simplify seeding and theme updates in the wizard. 

We now:

- seed only one theme, named "Default" (previously "Light")
- seed a user-selectable Dark color scheme
- rename the "Themes" wizard step to "Colors"
- update the default theme's color scheme if a default is set
(a new theme is created if there is no default)

spec/serializers/site_serializer_spec.rb

@@ -30,12 +30,9 @@ describe SiteSerializer do
   end
 
   it "includes user-selectable color schemes" do
-    scheme = ColorScheme.create_from_base(name: "Neutral", base_scheme_id: "Neutral")
-    scheme.user_selectable = true
-    scheme.save!
-
+    # it includes seeded color schemes
     serialized = described_class.new(Site.new(guardian), scope: guardian, root: false).as_json
-    expect(serialized[:user_color_schemes].count).to eq (1)
+    expect(serialized[:user_color_schemes].count).to eq(1)
 
     dark_scheme = ColorScheme.create_from_base(name: "ADarkScheme", base_scheme_id: "Dark")
     dark_scheme.user_selectable = true