Add batched background migration information to admin panel

The current admin panel in the app displays information about the status of background jobs. With the addition of the batched background migration framework, this panel will no longer capture the full status of ongoing background migrations.

Additionally, as part of our no-downtime upgrade process, we recommend for background migrations to be finished before beginning the upgrade. If the jobs do not have time to properly finish through sidekiq, any cleanup jobs will execute inline when post-deployment migrations are run.

In order to roll out primary key conversion migrations to self-hosted installations, we should update the panel to also include information about these migrations. This will also be helpful for us to monitor the status of ongoing migrations on GitLab.com.

Items left todo

• Add the retry button to failed migrations