Extract User from Actors context

Mobilizon.Actors.User -> Mobilizon.Users.User
Also Mobilizon.Actors.Service now become Mobilizon.User.Service
And Mobilizon.Users and Mobilizon.UsersTest is introduced.

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

lib/mobilizon/email/user.ex

@@ -2,7 +2,7 @@ defmodule Mobilizon.Email.User do
   @moduledoc """
   Handles emails sent to users
   """
-  alias Mobilizon.Actors.User
+  alias Mobilizon.Users.User
 
   import Bamboo.Email
   import Bamboo.Phoenix