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

db/migrate/20181127130500_identity_id_to_bigint.rb

@@ -0,0 +1,27 @@
+require Rails.root.join('lib', 'mastodon', 'migration_helpers')
+
+class IdentityIdToBigint < ActiveRecord::Migration[5.2]
+  include Mastodon::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def up
+    safety_assured do
+      change_column_type_concurrently :identities, :id, :bigint
+      cleanup_concurrent_column_type_change :identities, :id
+
+      change_column_type_concurrently :identities, :user_id, :bigint
+      cleanup_concurrent_column_type_change :identities, :user_id
+    end
+  end
+
+  def down
+    safety_assured do
+      change_column_type_concurrently :identities, :id, :integer
+      cleanup_concurrent_column_type_change :identities, :id
+
+      change_column_type_concurrently :identities, :user_id, :integer
+      cleanup_concurrent_column_type_change :identities, :user_id
+    end
+  end
+end