Different colors for new tracks, types and levels

Every time a new track, event type or difficulty level is created, the color picker has a different color by default.

Closes https://github.com/openSUSE/osem/issues/1077

app/controllers/admin/difficulty_levels_controller.rb

@@ -11,7 +11,7 @@ module Admin
     def edit; end
 
     def new
-      @difficulty_level = @conference.program.difficulty_levels.new
+      @difficulty_level = @conference.program.difficulty_levels.new(color: @conference.next_color_for_collection(:levels))
     end
 
     def create