Fix locked milestone in boards being remove-able

What does this MR do?

CE backport, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11696

• Remove "x" button from locked milestone
• Refactor some side-effects into a setup function (so this.cantedit is in place)

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

Why was this MR needed?

• When visiting a board that is associated with a milestone, the locked visual milestone has a usable "x" remove button.

Screenshots (if relevant)

http://localhost:3010/gitlab-org/gitlab-ce/boards/2?milestone_title=v4.0

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #2433 (closed)