Periodically mark projects that are stuck in importing as failed

What does this MR do?

Marks stuck project imports as failed. Implements: https://gitlab.com/gitlab-org/gitlab-ce/issues/17709#note_24583548

Are there points in the code the reviewer needs to double check?

There is one compromise. As stated in https://gitlab.com/gitlab-org/gitlab-ce/issues/17709#note_24583548, with this approach, we won't effectively kill the running process (but at least we'll mark the project as failed if they are not actually running, or we suspect they should not be).

The expiration time is set to 10 hours instead of 30 minutes for RepositoryImportWorker - Not sure whether that's too long or too short.

Why was this MR needed?

Projects can remain in started status even though they finished.

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?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/17709