mailers: add procedure context to the confirmation link

This allows to redirect the user to the procedure they signed up for
even when the browser session is not available (like if they changed
of browser).

Fix #4738

app/mailers/devise_user_mailer.rb

@@ -15,6 +15,7 @@ class DeviseUserMailer < Devise::Mailer
 
   def confirmation_instructions(record, token, opts = {})
     opts[:from] = NO_REPLY_EMAIL
+    @procedure = CurrentConfirmation.procedure_after_confirmation || nil
     super
   end
 end