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

Add webpush notification permission in frontend

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

db/migrate/20200504144712_add_index_on_notification_subscriptions.rb

@@ -0,0 +1,6 @@
+class AddIndexOnNotificationSubscriptions < ActiveRecord::Migration[6.0]
+  def change
+    add_column :notification_subscriptions, :identifier, :string
+    add_index :notification_subscriptions, :identifier, unique: true
+  end
+end