Change models, new migrations, fix front and make tests work

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

priv/repo/migrations/20180112100000_create_tracks.exs

@@ -0,0 +1,16 @@
+defmodule Eventos.Repo.Migrations.CreateTracks do
+  use Ecto.Migration
+
+  def change do
+    create table(:tracks) do
+      add :name, :string
+      add :description, :text
+      add :color, :string
+
+      add :event_id, references(:events, on_delete: :delete_all), null: false
+
+      timestamps()
+    end
+
+  end
+end