Save track files in new Sound model:

* Consolidate migrations
* Add Sound model. Each sound can belong to a track and contains
  informations about one sound file.

app/models/sound.rb

@@ -0,0 +1,18 @@
+require 'fileutils'
+
+class Sound < ActiveRecord::Base
+  belongs_to :track
+
+  validates_presence_of :sha256
+  validates_presence_of :mime_type
+
+  def path
+    "#{Rails.root}/data/sounds/#{sha256}"
+  end
+
+  def file=(file)
+    self.sha256 = Digest::SHA256.file(file.path).hexdigest
+    FileUtils.cp file.path, path
+    self.mime_type = file.content_type
+  end
+end