WIP: fix tests when mr_vue_discussions cookie not set

What does this MR do?

Start making mr_vue_discussions changes backwards compatible

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

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?