[ci skip] Update migration to be reversible and use add_column_with_default

db/migrate/20170428123910_add_clientside_sentry_to_application_settings.rb

-# rubocop:disable all
 class AddClientsideSentryToApplicationSettings < ActiveRecord::Migration
-  DOWNTIME = true
-  DOWNTIME_REASON = 'This migration requires downtime because we must add 2 new columns, 1 of which has a default value.'
-
-  def change
-    change_table :application_settings do |t|
-      t.boolean :clientside_sentry_enabled, default: false
-      t.string :clientside_sentry_dsn
-    end
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  # When a migration requires downtime you **must** uncomment the following
+  # constant and define a short and easy to understand explanation as to why the
+  # migration requires downtime.
+  # DOWNTIME_REASON = ''
+
+  # When using the methods "add_concurrent_index" or "add_column_with_default"
+  # you must disable the use of transactions as these methods can not run in an
+  # existing transaction. When using "add_concurrent_index" make sure that this
+  # method is the _only_ method called in the migration, any other changes
+  # should go in a separate migration. This ensures that upon failure _only_ the
+  # index creation fails and can be retried or reverted easily.
+  #
+  # To disable transactions uncomment the following line and remove these
+  # comments:
+  disable_ddl_transaction!
+
+  def up
+    add_column_with_default :application_settings, :clientside_sentry_enabled, :boolean, default: false
+    add_column :application_settings, :clientside_sentry_dsn, :string
+  end
+
+  def down
+    remove_columns :application_settings, :clientside_sentry_enabled, :clientside_sentry_dsn
   end
 end