Fix unconfirmed users clean-up issues [SCI-3172]

app/controllers/users/omniauth_callbacks_controller.rb

@@ -58,6 +58,8 @@ module Users
                             reason: I18n.t('devise.linkedin.failed_to_save'))
           redirect_to after_omniauth_failure_path_for(resource_name) and return
         end
+        # Confirm user
+        @user.update_column(:confirmed_at, @user.created_at)
         redirect_to users_sign_up_provider_path(user: @user)
       end
     end

app/models/user.rb

@@ -209,7 +209,7 @@ class User < ApplicationRecord
 
   has_many :user_notifications, inverse_of: :user
   has_many :notifications, through: :user_notifications
-  has_many :user_system_notifications
+  has_many :user_system_notifications, dependent: :destroy
   has_many :system_notifications, through: :user_system_notifications
   has_many :zip_exports, inverse_of: :user, dependent: :destroy
   has_many :datatables_teams, class_name: '::Views::Datatables::DatatablesTeam'