Change identities id column to a bigint (#9371)

* fix: change Identity's id column to a bigint

This appears to be the last model created using a 5.0 migration,
where column types defaulted to `integer` rather than `bigint`.

This migration changes the column type to match that of all of the
other ID columns.

* Change user_id column in identities to bigint and fix down-migration

app/models/identity.rb

@@ -3,12 +3,12 @@
 #
 # Table name: identities
 #
-#  id         :integer          not null, primary key
-#  user_id    :integer
 #  provider   :string           default(""), not null
 #  uid        :string           default(""), not null
 #  created_at :datetime         not null
 #  updated_at :datetime         not null
+#  id         :bigint(8)        not null, primary key
+#  user_id    :bigint(8)
 #
 
 class Identity < ApplicationRecord