Feature: Add web push notification permission in frontend (#766)

Add webpush notification permission in frontend

Co-authored-by: Sojan <sojan@pepalo.com>

app/models/notification_subscription.rb

@@ -3,6 +3,7 @@
 # Table name: notification_subscriptions
 #
 #  id                      :bigint           not null, primary key
+#  identifier              :string
 #  subscription_attributes :jsonb            not null
 #  subscription_type       :integer          not null
 #  created_at              :datetime         not null
@@ -11,11 +12,13 @@
 #
 # Indexes
 #
-#  index_notification_subscriptions_on_user_id  (user_id)
+#  index_notification_subscriptions_on_identifier  (identifier) UNIQUE
+#  index_notification_subscriptions_on_user_id     (user_id)
 #
 
 class NotificationSubscription < ApplicationRecord
   belongs_to :user
+  validates :identifier, presence: true
 
   SUBSCRIPTION_TYPES = {
     browser_push: 1,