Add post-deployment migration system (#8182)

Adopted from GitLab CE. Generate new migration with:

    rails g post_deployment_migration name_of_migration_here

By default they are run together with db:migrate. To not run them,
the env variable SKIP_POST_DEPLOYMENT_MIGRATIONS must be set

Code by Yorick Peterse <yorickpeterse@gmail.com>, see also:

83c8241160

lib/generators/post_deployment_migration_generator.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require 'rails/generators'
+
+module Rails
+  class PostDeploymentMigrationGenerator < Rails::Generators::NamedBase
+    def create_migration_file
+      timestamp = Time.zone.now.strftime('%Y%m%d%H%M%S')
+
+      template 'migration.rb', "db/post_migrate/#{timestamp}_#{file_name}.rb"
+    end
+
+    def migration_class_name
+      file_name.camelize
+    end
+  end
+end