Improves handling of stuck projects

What does this MR do?

Improves the failure handling of stuck projects in general

Relevant checklist

• Detect stuck mirrors using the same mechanism we use to detect stuck initial imports, which actually checks whether the job ID is still on the Sidekiq queue. A mirror that is still queued is never stuck. We need to set the SidekiqStatus timeout appropriately, of course.

• Store that JID in the DB at schedule time, not at start time (like initial imports do right now)

• Use that mechanism for forks too, for consistency

• Treat projects that are in state started but don't have a JID as stuck

• Fail early from import/fork/mirror workers when state is not actually scheduled (or none?)

• Reset import_error at finish, NOT at start, since we still want to display it during an update

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #3108 (closed)