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

spec/factories/event_types.rb

@@ -6,6 +6,7 @@ FactoryGirl.define do
     length 30
     minimum_abstract_length 0
     maximum_abstract_length 500
+    color '#ffffff'
     program
   end