Allow Bitbucket Server migration to be easily canceled by Admin

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BB Server issue for epic Allow importers to be easily stopped (&12427).

Proposed actions

• Implement a mechanism so that Admin can easily cancel a BB Server migrations. The aim is to allow an instance Admin to react fast to an incident - to cancel and delete the import. "Admin" means the instance Admin, not the user performing the migration.
• Document in the runbook how to do that. Also, include in the runbook that importers can be disabled via UI.