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

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

priv/repo/migrations/20180109150000_prerequites.exs

@@ -0,0 +1,16 @@
+defmodule Eventos.Repo.Migrations.Prerequites do
+  use Ecto.Migration
+
+  def up do
+    execute """
+    CREATE TYPE datetimetz AS (
+        dt timestamptz,
+        tz varchar
+    );
+    """
+  end
+
+  def down do
+    execute "DROP TYPE IF EXISTS datetimetz;"
+  end
+end