Add REST API for Web Push Notifications subscriptions (#7445)

- POST /api/v1/push/subscription
- PUT /api/v1/push/subscription
- DELETE /api/v1/push/subscription
- New OAuth scope: "push" (required for the above methods)

db/migrate/20180510230049_migrate_web_push_subscriptions.rb

@@ -0,0 +1,13 @@
+class MigrateWebPushSubscriptions < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    add_index :web_push_subscriptions, :user_id, algorithm: :concurrently
+    add_index :web_push_subscriptions, :access_token_id, algorithm: :concurrently
+  end
+
+  def down
+    remove_index :web_push_subscriptions, :user_id
+    remove_index :web_push_subscriptions, :access_token_id
+  end
+end