Expose finalize BBM command in UI

What does this MR do and why?

Extracted from: !129735 (comment 1519331741)

We have the script format specified in the upgrade docs to finalize BBMs manually, but filling in the params can be tricky sometimes with necessary job_arguments and class name. So this MR aims to expose an option in Admin UI to copy the command to manually finalize.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Make a Gitlab::Database::BackgroundMigration::BatchedMigration record to have failed status (status: 4)
2. Go to http://gdk.test:3000/admin/background_migrations?tab=failed tab.
3. On clicking the Copy command dropdown, you should be able to get fully formed script.
   1. eg: sudo gitlab-rake gitlab:background_migrations:finalize[CopyColumnUsingBackgroundMigrationJob,events,id,'[[\"column_1\"]\\,[\"column_1_convert_to_bigint\"]]']

MR acceptance checklist

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Related to #422572 (closed)