Remove hack from milestone_select used to access boardsStore

The following discussion from gitlab-ce!22210 should be addressed:

• @mikegreiling started a discussion: (+2 comments)

  Note: These functions were merely moved from app/assets/javascripts/boards/index.js. This hack is not new to this refactoring. These should be resolved/removed, but perhaps in another MR since this one is already quite large.