This commit includes migrations to backfill environment_id on
deployment_merge_requests table.

Environment_id is nullable and was introduced to deal with duplicated
environments in MR deployment tracking
https://gitlab.com/gitlab-org/gitlab/issues/199256

Here we backfill the old entries that still have a NULL environment_id

We batch entries with ranges of merge_request_id and schedule some
background migrations.

The migration will delete duplicates and backfill environment_id
Co-authored-by: Yorick Peterse <yorick@yorickpeterse.com>