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/models/track.rb

@@ -4,6 +4,7 @@ class Track < ActiveRecord::Base
 
   before_create :generate_guid
   validates :name, presence: true
+  validates :color, format: /\A#[0-9a-fA-F]{6}\z/
 
   private