Extract quarantine code and add tests

What does this MR do?

It wasn't possible to test the quarantine feature as implemented fully in the RSpec configuration because of restrictions on running isolated contexts within other contexts (i.e., using RSpec to test itself)

This extracts the quarantine code into a module and adds unit tests that test the feature within a sandbox, thereby allowing tests within tests.

TODO:

• add more tests
• target master after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24942 is merged

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/57666

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer