Fix locked milestone in boards being remove-able

What does this MR do?

CE backport,

  • 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)


Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #2433 (closed)

Edited by username-removed-892863

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