Backport Fix locked milestone in boards being remove-able.
See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1963

Fix https://gitlab.com/gitlab-org/gitlab-ee/issues/2433

Conflicts:
	app/assets/javascripts/boards/boards_bundle.js
	app/assets/javascripts/boards/filtered_search_boards.js
	app/assets/javascripts/filtered_search/filtered_search_manager.js
	spec/features/boards/boards_spec.rb