Resolve "Add issue weight indicator on the issue cards in the issue board"

What does this MR do?

Adds the issue weight indicator on the issue cards in the issue board view.

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

Ruby tests since I've updated the issue endpoint response payload.

Why was this MR needed?

Makes it much quicker to get an overview and compare the complexity of issues.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • 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
• 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
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #4006 (closed)