Fix loading spinner for the weight block in the Issue Boards

What does this MR do?

• Fix loading spinner for the weight block in the Issue Boards

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

Why was this MR needed?

Spinner was working in deliverable MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3566 but this regressed in a CE->EE merge where git didn't consider app/assets/javascripts/boards/models/issue.js as a conflict so it wasn't double-checked.

Ideally we would have better separation to avoid the conflict, https://gitlab.com/gitlab-org/gitlab-ee/issues/4246

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
  • We could add an end-to-end test but doesn't seem quite worth it 🤷 It's more of a problem with Vue not watching the property if it isn't there to begin with. We could also use Vue.set in setLoadingState.
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #41178 https://gitlab.com/gitlab-org/gitlab-ee/issues/4363