Backfill remaining namespace traversal_ids

What does this MR do?

The first background migration job to backfill namespaces.traversal_ids did not cover all namespaces. This is most likely due to a misunderstanding we had between the relationship of chatops and default feature flag enablement.

This MR schedules the same background migrations as from the first background migration. Namely Gitlab::BackgroundMigration::BackfillNamespaceTraversalIdsRoots and Gitlab::BackgroundMigration::BackfillNamespaceTraversalIdsChildren.

$ rails db:migrate                                                                                                                                                  == 20210611062115 BackfillRemainingTraversalIds: migrating ====================
-- Scheduled 0 BackfillNamespaceTraversalIdsRoots jobs with a maximum of 1000 records per batch and an interval of 120 seconds.

The migration is expected to take at least 0 seconds. Expect all jobs to have completed after 2021-06-11 06:53:53 UTC."
-- Scheduled 0 BackfillNamespaceTraversalIdsChildren jobs with a maximum of 1000 records per batch and an interval of 120 seconds.

The migration is expected to take at least 120 seconds. Expect all jobs to have completed after 2021-06-11 06:55:53 UTC."
== 20210611062115 BackfillRemainingTraversalIds: migrated (0.0352s) ===========


$ rails db:rollback                                                                                                                                                 == 20210611062115 BackfillRemainingTraversalIds: reverting ====================
== 20210611062115 BackfillRemainingTraversalIds: reverted (0.0000s) ===========

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