[day-05] preload training alongside user

Preload training and exercises when fetching an user and expose this
information in graphql.

lib/wabanex_web/schema/types/user.ex

@@ -6,6 +6,7 @@ defmodule WabanexWeb.Schema.Types.User do
     field :id, non_null(:uuid4)
     field :name, non_null(:string)
     field :email, non_null(:string)
+    field :trainings, list_of(:training)
   end
 
   input_object :create_user_input do