Improve the way the instance name is injected into emails

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

lib/web/email/user.ex

@@ -172,19 +172,17 @@ defmodule Mobilizon.Web.Email.User do
         _locale \\ "en"
       ) do
     Gettext.put_locale(user_locale)
-    instance_name = Config.instance_name()
 
     subject =
       gettext(
         "Mobilizon on %{instance}: confirm your email address",
-        instance: instance_name
+        instance: Config.instance_name()
       )
 
     Email.base_email(to: unconfirmed_email, subject: subject)
     |> assign(:locale, user_locale)
     |> assign(:subject, subject)
     |> assign(:token, confirmation_token)
-    |> assign(:instance_name, instance_name)
     |> render(:email_changed_new)
   end