Fix deprecation: returning false in Active Record and Active Model callbacks

What does this MR do?

Fixes deprecation: Returning `false` in Active Record and Active Model callbacks will not implicitly halt a callback chain in Rails 5.1. To explicitly halt the callback chain, please use `throw :abort` instead. (called from block (3 levels) in <top (required)> at /builds/jlemaes/gitlab-ce/spec/models/list_spec.rb:36)

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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