Add retry / resync to project importer

Problem to solve

Imports can take a very long time to run and will sometimes fail, such as when the StuckImportWorker kills the job.

Intended users

Systems Administrator / DevOps / Site administrator.

Users moving to GitLab from other version control software.

Further details

@ashmckenzie wrote a script that allowed an import to finish. We believe this can be implemented as a retry feature.

Proposal

Requires discussion on how best to implement this feature.

Permissions and Security

Documentation

What does success look like, and how can we measure that?

Links / references

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/56555

Related customer issue: https://gitlab.zendesk.com/agent/tickets/117496 (Internal only)