Resolve "Board with config API"

CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20364

What does this MR do?

Add API endpoint for viewing and editing board config

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

I added authorize in the lib/api/boards.rb:36 which I think was missed out previously.

Why was this MR needed?

Enable configuring board with API.

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
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• 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 #4266 (closed)