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/migrate/20200917192924_add_notify_to_follows.rb

@@ -0,0 +1,19 @@
+require Rails.root.join('lib', 'mastodon', 'migration_helpers')
+
+class AddNotifyToFollows < ActiveRecord::Migration[5.1]
+  include Mastodon::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def up
+    safety_assured do
+      add_column_with_default :follows, :notify, :boolean, default: false, allow_null: false
+      add_column_with_default :follow_requests, :notify, :boolean, default: false, allow_null: false
+    end
+  end
+
+  def down
+    remove_column :follows, :notify
+    remove_column :follow_requests, :notify
+  end
+end