Re-add template column to services table

We attempted to revert the rename of the `template` to `instance` column
in the `services` table in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/24857 and
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/24885, but for
some installations (e.g. staging), the `template` column had been
dropped already because post-deploy migrations had finished.

We never actually shipped this change to production, but for these
environments we need to re-add the `template` column back into the table
or project creation will fail.

Note we do not do a data migration (e.g. set the `template` column to
true for rows that have a NULL `project_id`) for two reasons:

1. We don't want to insert a template by accident.
2. This rename only finished on development/staging environments.

Note that this migration happens in a post-deploy migration to ensure
`20200211152410_remove_instance_from_services.rb` gets a chance
to undo the migration first.

db/post_migrate/20200212052620_readd_template_column_to_services.rb

+# frozen_string_literal: true
+
+class ReaddTemplateColumnToServices < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    return if column_exists? :services, :template
+
+    # The migration to drop the template column never actually shipped
+    # to production, so we should be okay to re-add it without worrying
+    # about doing a data migration.  If we needed to restore the value
+    # of `template`, we would look for entries with `project_id IS NULL`.
+    add_column_with_default :services, :template, :boolean, default: false, allow_null: true
+  end
+
+  def down
+    # NOP since the column is expected to exist
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2020_02_11_152410) do
+ActiveRecord::Schema.define(version: 2020_02_12_052620) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"