Add option to be notified when a followed user posts (#13546)

* Add bell button

Fix #4890

* Remove duplicate type from post-deployment migration

* Fix legacy class type mappings

* Improve query performance with better index

* Fix validation

* Remove redundant index from notifications

db/post_migrate/20200917193528_migrate_notifications_type.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class MigrateNotificationsType < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  TYPES_TO_MIGRATE = {
+    'Mention'       => :mention,
+    'Status'        => :reblog,
+    'Follow'        => :follow,
+    'FollowRequest' => :follow_request,
+    'Favourite'     => :favourite,
+    'Poll'          => :poll,
+  }.freeze
+
+  def up
+    TYPES_TO_MIGRATE.each_pair do |activity_type, type|
+      Notification.where(activity_type: activity_type, type: nil).in_batches.update_all(type: type)
+    end
+  end
+
+  def down; end
+end

db/post_migrate/20200917222734_remove_index_notifications_on_account_activity.rb

@@ -0,0 +1,15 @@
+# frozen_string_literal: true
+
+class RemoveIndexNotificationsOnAccountActivity < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    remove_index :notifications, name: :account_activity
+    remove_index :notifications, name: :index_notifications_on_account_id_and_id
+  end
+
+  def down
+    add_index :notifications, [:account_id, :activity_id, :activity_type], unique: true, name: 'account_activity', algorithm: :concurrently
+    add_index :notifications, [:account_id, :id], order: { id: :desc }, algorithm: :concurrently
+  end
+end